The HI Properties and Environment of Lyman-alpha Absorbers

高三宇宙奥秘英语阅读理解30题1<背景文章>Black holes are one of the most fascinating and mysterious phenomena in the universe. A black hole is formed when a massive star collapses at the end of its life. The gravitational pull of a black hole is so strong that nothing, not even light, can escape from it.The formation of a black hole begins with the collapse of a massive star. As the star runs out of fuel, it can no longer support its own weight and begins to collapse. The collapse continues until the star reaches a critical density, at which point it becomes a black hole.Black holes have several unique characteristics. One of the most notable is their event horizon, which is the boundary beyond which nothing can escape. Another characteristic is their intense gravitational field, which can distort the space and time around them.Black holes can have a significant impact on the surrounding celestial bodies. They can attract and swallow nearby stars and planets, and their gravitational pull can also affect the orbits of other celestial bodies.Scientists are still working to understand black holes better. They use a variety of tools and techniques, such as telescopes and computer simulations, to study these mysterious objects. Despite significant progressin recent years, there is still much that we don't know about black holes.1. What is a black hole formed by?A. A small star collapsing.B. A massive star collapsing.C. A planet collapsing.D. A moon collapsing.答案：B。

DOI: 10.1126/science.1094786, 441 (2004);304Science et al.Mitchell S. Abrahamsen,Cryptosporidium parvum Complete Genome Sequence of the Apicomplexan, (this information is current as of October 7, 2009 ):The following resources related to this article are available online at/cgi/content/full/304/5669/441version of this article at:including high-resolution figures, can be found in the online Updated information and services,/cgi/content/full/1094786/DC1 can be found at:Supporting Online Material/cgi/content/full/304/5669/441#otherarticles , 9 of which can be accessed for free: cites 25 articles This article 239 article(s) on the ISI Web of Science. cited by This article has been /cgi/content/full/304/5669/441#otherarticles 53 articles hosted by HighWire Press; see: cited by This article has been/cgi/collection/genetics Genetics: subject collections This article appears in the following/about/permissions.dtl in whole or in part can be found at: this article permission to reproduce of this article or about obtaining reprints Information about obtaining registered trademark of AAAS.is a Science 2004 by the American Association for the Advancement of Science; all rights reserved. The title Copyright American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the Science o n O c t o b e r 7, 2009w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o m3.R.Jackendoff,Foundations of Language:Brain,Gram-mar,Evolution(Oxford Univ.Press,Oxford,2003).4.Although for Frege(1),reference was established rela-tive to objects in the world,here we follow Jackendoff’s suggestion(3)that this is done relative to objects and the state of affairs as mentally represented.5.S.Zola-Morgan,L.R.Squire,in The Development andNeural Bases of Higher Cognitive Functions(New York Academy of Sciences,New York,1990),pp.434–456.6.N.Chomsky,Reflections on Language(Pantheon,New York,1975).7.J.Katz,Semantic Theory(Harper&Row,New York,1972).8.D.Sperber,D.Wilson,Relevance(Harvard Univ.Press,Cambridge,MA,1986).9.K.I.Forster,in Sentence Processing,W.E.Cooper,C.T.Walker,Eds.(Erlbaum,Hillsdale,NJ,1989),pp.27–85.10.H.H.Clark,Using Language(Cambridge Univ.Press,Cambridge,1996).11.Often word meanings can only be fully determined byinvokingworld knowledg e.For instance,the meaningof “flat”in a“flat road”implies the absence of holes.However,in the expression“aflat tire,”it indicates the presence of a hole.The meaningof“finish”in the phrase “Billfinished the book”implies that Bill completed readingthe book.However,the phrase“the g oatfin-ished the book”can only be interpreted as the goat eatingor destroyingthe book.The examples illustrate that word meaningis often underdetermined and nec-essarily intertwined with general world knowledge.In such cases,it is hard to see how the integration of lexical meaning and general world knowledge could be strictly separated(3,31).12.W.Marslen-Wilson,C.M.Brown,L.K.Tyler,Lang.Cognit.Process.3,1(1988).13.ERPs for30subjects were averaged time-locked to theonset of the critical words,with40items per condition.Sentences were presented word by word on the centerof a computer screen,with a stimulus onset asynchronyof600ms.While subjects were readingthe sentences,their EEG was recorded and amplified with a high-cut-off frequency of70Hz,a time constant of8s,and asamplingfrequency of200Hz.14.Materials and methods are available as supportingmaterial on Science Online.15.M.Kutas,S.A.Hillyard,Science207,203(1980).16.C.Brown,P.Hagoort,J.Cognit.Neurosci.5,34(1993).17.C.M.Brown,P.Hagoort,in Architectures and Mech-anisms for Language Processing,M.W.Crocker,M.Pickering,C.Clifton Jr.,Eds.(Cambridge Univ.Press,Cambridge,1999),pp.213–237.18.F.Varela et al.,Nature Rev.Neurosci.2,229(2001).19.We obtained TFRs of the single-trial EEG data by con-volvingcomplex Morlet wavelets with the EEG data andcomputingthe squared norm for the result of theconvolution.We used wavelets with a7-cycle width,with frequencies ranging from1to70Hz,in1-Hz steps.Power values thus obtained were expressed as a per-centage change relative to the power in a baselineinterval,which was taken from150to0ms before theonset of the critical word.This was done in order tonormalize for individual differences in EEG power anddifferences in baseline power between different fre-quency bands.Two relevant time-frequency compo-nents were identified:(i)a theta component,rangingfrom4to7Hz and from300to800ms after wordonset,and(ii)a gamma component,ranging from35to45Hz and from400to600ms after word onset.20.C.Tallon-Baudry,O.Bertrand,Trends Cognit.Sci.3,151(1999).tner et al.,Nature397,434(1999).22.M.Bastiaansen,P.Hagoort,Cortex39(2003).23.O.Jensen,C.D.Tesche,Eur.J.Neurosci.15,1395(2002).24.Whole brain T2*-weighted echo planar imaging bloodoxygen level–dependent(EPI-BOLD)fMRI data wereacquired with a Siemens Sonata1.5-T magnetic reso-nance scanner with interleaved slice ordering,a volumerepetition time of2.48s,an echo time of40ms,a90°flip angle,31horizontal slices,a64ϫ64slice matrix,and isotropic voxel size of3.5ϫ3.5ϫ3.5mm.For thestructural magnetic resonance image,we used a high-resolution(isotropic voxels of1mm3)T1-weightedmagnetization-prepared rapid gradient-echo pulse se-quence.The fMRI data were preprocessed and analyzedby statistical parametric mappingwith SPM99software(http://www.fi/spm99).25.S.E.Petersen et al.,Nature331,585(1988).26.B.T.Gold,R.L.Buckner,Neuron35,803(2002).27.E.Halgren et al.,J.Psychophysiol.88,1(1994).28.E.Halgren et al.,Neuroimage17,1101(2002).29.M.K.Tanenhaus et al.,Science268,1632(1995).30.J.J.A.van Berkum et al.,J.Cognit.Neurosci.11,657(1999).31.P.A.M.Seuren,Discourse Semantics(Basil Blackwell,Oxford,1985).32.We thank P.Indefrey,P.Fries,P.A.M.Seuren,and M.van Turennout for helpful discussions.Supported bythe Netherlands Organization for Scientific Research,grant no.400-56-384(P.H.).Supporting Online Material/cgi/content/full/1095455/DC1Materials and MethodsFig.S1References and Notes8January2004;accepted9March2004Published online18March2004;10.1126/science.1095455Include this information when citingthis paper.Complete Genome Sequence ofthe Apicomplexan,Cryptosporidium parvumMitchell S.Abrahamsen,1,2*†Thomas J.Templeton,3†Shinichiro Enomoto,1Juan E.Abrahante,1Guan Zhu,4 Cheryl ncto,1Mingqi Deng,1Chang Liu,1‡Giovanni Widmer,5Saul Tzipori,5GregoryA.Buck,6Ping Xu,6 Alan T.Bankier,7Paul H.Dear,7Bernard A.Konfortov,7 Helen F.Spriggs,7Lakshminarayan Iyer,8Vivek Anantharaman,8L.Aravind,8Vivek Kapur2,9The apicomplexan Cryptosporidium parvum is an intestinal parasite that affects healthy humans and animals,and causes an unrelenting infection in immuno-compromised individuals such as AIDS patients.We report the complete ge-nome sequence of C.parvum,type II isolate.Genome analysis identifies ex-tremely streamlined metabolic pathways and a reliance on the host for nu-trients.In contrast to Plasmodium and Toxoplasma,the parasite lacks an api-coplast and its genome,and possesses a degenerate mitochondrion that has lost its genome.Several novel classes of cell-surface and secreted proteins with a potential role in host interactions and pathogenesis were also detected.Elu-cidation of the core metabolism,including enzymes with high similarities to bacterial and plant counterparts,opens new avenues for drug development.Cryptosporidium parvum is a globally impor-tant intracellular pathogen of humans and animals.The duration of infection and patho-genesis of cryptosporidiosis depends on host immune status,ranging from a severe but self-limiting diarrhea in immunocompetent individuals to a life-threatening,prolonged infection in immunocompromised patients.Asubstantial degree of morbidity and mortalityis associated with infections in AIDS pa-tients.Despite intensive efforts over the past20years,there is currently no effective ther-apy for treating or preventing C.parvuminfection in humans.Cryptosporidium belongs to the phylumApicomplexa,whose members share a com-mon apical secretory apparatus mediating lo-comotion and tissue or cellular invasion.Many apicomplexans are of medical or vet-erinary importance,including Plasmodium,Babesia,Toxoplasma,Neosprora,Sarcocys-tis,Cyclospora,and Eimeria.The life cycle ofC.parvum is similar to that of other cyst-forming apicomplexans(e.g.,Eimeria and Tox-oplasma),resulting in the formation of oocysts1Department of Veterinary and Biomedical Science,College of Veterinary Medicine,2Biomedical Genom-ics Center,University of Minnesota,St.Paul,MN55108,USA.3Department of Microbiology and Immu-nology,Weill Medical College and Program in Immu-nology,Weill Graduate School of Medical Sciences ofCornell University,New York,NY10021,USA.4De-partment of Veterinary Pathobiology,College of Vet-erinary Medicine,Texas A&M University,College Sta-tion,TX77843,USA.5Division of Infectious Diseases,Tufts University School of Veterinary Medicine,NorthGrafton,MA01536,USA.6Center for the Study ofBiological Complexity and Department of Microbiol-ogy and Immunology,Virginia Commonwealth Uni-versity,Richmond,VA23198,USA.7MRC Laboratoryof Molecular Biology,Hills Road,Cambridge CB22QH,UK.8National Center for Biotechnology Infor-mation,National Library of Medicine,National Insti-tutes of Health,Bethesda,MD20894,USA.9Depart-ment of Microbiology,University of Minnesota,Min-neapolis,MN55455,USA.*To whom correspondence should be addressed.E-mail:abe@†These authors contributed equally to this work.‡Present address:Bioinformatics Division,Genetic Re-search,GlaxoSmithKline Pharmaceuticals,5MooreDrive,Research Triangle Park,NC27009,USA.R E P O R T S SCIENCE VOL30416APRIL2004441o n O c t o b e r 7 , 2 0 0 9 w w w . s c i e n c e m a g . o r g D o w n l o a d e d f r o mthat are shed in the feces of infected hosts.C.parvum oocysts are highly resistant to environ-mental stresses,including chlorine treatment of community water supplies;hence,the parasite is an important water-and food-borne pathogen (1).The obligate intracellular nature of the par-asite ’s life cycle and the inability to culture the parasite continuously in vitro greatly impair researchers ’ability to obtain purified samples of the different developmental stages.The par-asite cannot be genetically manipulated,and transformation methodologies are currently un-available.To begin to address these limitations,we have obtained the complete C.parvum ge-nome sequence and its predicted protein com-plement.(This whole-genome shotgun project has been deposited at DDBJ/EMBL/GenBank under the project accession AAEE00000000.The version described in this paper is the first version,AAEE01000000.)The random shotgun approach was used to obtain the complete DNA sequence (2)of the Iowa “type II ”isolate of C.parvum .This isolate readily transmits disease among numerous mammals,including humans.The resulting ge-nome sequence has roughly 13ϫgenome cov-erage containing five gaps and 9.1Mb of totalDNA sequence within eight chromosomes.The C.parvum genome is thus quite compact rela-tive to the 23-Mb,14-chromosome genome of Plasmodium falciparum (3);this size difference is predominantly the result of shorter intergenic regions,fewer introns,and a smaller number of genes (Table 1).Comparison of the assembled sequence of chromosome VI to that of the recently published sequence of chromosome VI (4)revealed that our assembly contains an ad-ditional 160kb of sequence and a single gap versus two,with the common sequences dis-playing a 99.993%sequence identity (2).The relative paucity of introns greatly simplified gene predictions and facilitated an-notation (2)of predicted open reading frames (ORFs).These analyses provided an estimate of 3807protein-encoding genes for the C.parvum genome,far fewer than the estimated 5300genes predicted for the Plasmodium genome (3).This difference is primarily due to the absence of an apicoplast and mitochondrial genome,as well as the pres-ence of fewer genes encoding metabolic functions and variant surface proteins,such as the P.falciparum var and rifin molecules (Table 2).An analysis of the encoded pro-tein sequences with the program SEG (5)shows that these protein-encoding genes are not enriched in low-complexity se-quences (34%)to the extent observed in the proteins from Plasmodium (70%).Our sequence analysis indicates that Cryptosporidium ,unlike Plasmodium and Toxoplasma ,lacks both mitochondrion and apicoplast genomes.The overall complete-ness of the genome sequence,together with the fact that similar DNA extraction proce-dures used to isolate total genomic DNA from C.parvum efficiently yielded mito-chondrion and apicoplast genomes from Ei-meria sp.and Toxoplasma (6,7),indicates that the absence of organellar genomes was unlikely to have been the result of method-ological error.These conclusions are con-sistent with the absence of nuclear genes for the DNA replication and translation machinery characteristic of mitochondria and apicoplasts,and with the lack of mito-chondrial or apicoplast targeting signals for tRNA synthetases.A number of putative mitochondrial pro-teins were identified,including components of a mitochondrial protein import apparatus,chaperones,uncoupling proteins,and solute translocators (table S1).However,the ge-nome does not encode any Krebs cycle en-zymes,nor the components constituting the mitochondrial complexes I to IV;this finding indicates that the parasite does not rely on complete oxidation and respiratory chains for synthesizing adenosine triphosphate (ATP).Similar to Plasmodium ,no orthologs for the ␥,␦,or εsubunits or the c subunit of the F 0proton channel were detected (whereas all subunits were found for a V-type ATPase).Cryptosporidium ,like Eimeria (8)and Plas-modium ,possesses a pyridine nucleotide tran-shydrogenase integral membrane protein that may couple reduced nicotinamide adenine dinucleotide (NADH)and reduced nico-tinamide adenine dinucleotide phosphate (NADPH)redox to proton translocation across the inner mitochondrial membrane.Unlike Plasmodium ,the parasite has two copies of the pyridine nucleotide transhydrogenase gene.Also present is a likely mitochondrial membrane –associated,cyanide-resistant alter-native oxidase (AOX )that catalyzes the reduction of molecular oxygen by ubiquinol to produce H 2O,but not superoxide or H 2O 2.Several genes were identified as involved in biogenesis of iron-sulfur [Fe-S]complexes with potential mitochondrial targeting signals (e.g.,nifS,nifU,frataxin,and ferredoxin),supporting the presence of a limited electron flux in the mitochondrial remnant (table S2).Our sequence analysis confirms the absence of a plastid genome (7)and,additionally,the loss of plastid-associated metabolic pathways including the type II fatty acid synthases (FASs)and isoprenoid synthetic enzymes thatTable 1.General features of the C.parvum genome and comparison with other single-celled eukaryotes.Values are derived from respective genome project summaries (3,26–28).ND,not determined.FeatureC.parvum P.falciparum S.pombe S.cerevisiae E.cuniculiSize (Mbp)9.122.912.512.5 2.5(G ϩC)content (%)3019.43638.347No.of genes 38075268492957701997Mean gene length (bp)excluding introns 1795228314261424ND Gene density (bp per gene)23824338252820881256Percent coding75.352.657.570.590Genes with introns (%)553.9435ND Intergenic regions (G ϩC)content %23.913.632.435.145Mean length (bp)5661694952515129RNAsNo.of tRNA genes 454317429944No.of 5S rRNA genes 6330100–2003No.of 5.8S ,18S ,and 28S rRNA units 57200–400100–20022Table parison between predicted C.parvum and P.falciparum proteins.FeatureC.parvum P.falciparum *Common †Total predicted proteins380752681883Mitochondrial targeted/encoded 17(0.45%)246(4.7%)15Apicoplast targeted/encoded 0581(11.0%)0var/rif/stevor ‡0236(4.5%)0Annotated as protease §50(1.3%)31(0.59%)27Annotated as transporter ࿣69(1.8%)34(0.65%)34Assigned EC function ¶167(4.4%)389(7.4%)113Hypothetical proteins925(24.3%)3208(60.9%)126*Values indicated for P.falciparum are as reported (3)with the exception of those for proteins annotated as protease or transporter.†TBLASTN hits (e Ͻ–5)between C.parvum and P.falciparum .‡As reported in (3).§Pre-dicted proteins annotated as “protease or peptidase”for C.parvum (CryptoGenome database,)and P.falciparum (PlasmoDB database,).࿣Predicted proteins annotated as “trans-porter,permease of P-type ATPase”for C.parvum (CryptoGenome)and P.falciparum (PlasmoDB).¶Bidirectional BLAST hit (e Ͻ–15)to orthologs with assigned Enzyme Commission (EC)numbers.Does not include EC assignment numbers for protein kinases or protein phosphatases (due to inconsistent annotation across genomes),or DNA polymerases or RNA polymerases,as a result of issues related to subunit inclusion.(For consistency,46proteins were excluded from the reported P.falciparum values.)R E P O R T S16APRIL 2004VOL 304SCIENCE 442 o n O c t o b e r 7, 2009w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o mare otherwise localized to the plastid in other apicomplexans.C.parvum fatty acid biosynthe-sis appears to be cytoplasmic,conducted by a large(8252amino acids)modular type I FAS (9)and possibly by another large enzyme that is related to the multidomain bacterial polyketide synthase(10).Comprehensive screening of the C.parvum genome sequence also did not detect orthologs of Plasmodium nuclear-encoded genes that contain apicoplast-targeting and transit sequences(11).C.parvum metabolism is greatly stream-lined relative to that of Plasmodium,and in certain ways it is reminiscent of that of another obligate eukaryotic parasite,the microsporidian Encephalitozoon.The degeneration of the mi-tochondrion and associated metabolic capabili-ties suggests that the parasite largely relies on glycolysis for energy production.The parasite is capable of uptake and catabolism of mono-sugars(e.g.,glucose and fructose)as well as synthesis,storage,and catabolism of polysac-charides such as trehalose and amylopectin. Like many anaerobic organisms,it economizes ATP through the use of pyrophosphate-dependent phosphofructokinases.The conver-sion of pyruvate to acetyl–coenzyme A(CoA) is catalyzed by an atypical pyruvate-NADPH oxidoreductase(Cp PNO)that contains an N-terminal pyruvate–ferredoxin oxidoreductase (PFO)domain fused with a C-terminal NADPH–cytochrome P450reductase domain (CPR).Such a PFO-CPR fusion has previously been observed only in the euglenozoan protist Euglena gracilis(12).Acetyl-CoA can be con-verted to malonyl-CoA,an important precursor for fatty acid and polyketide biosynthesis.Gly-colysis leads to several possible organic end products,including lactate,acetate,and ethanol. The production of acetate from acetyl-CoA may be economically beneficial to the parasite via coupling with ATP production.Ethanol is potentially produced via two in-dependent pathways:(i)from the combination of pyruvate decarboxylase and alcohol dehy-drogenase,or(ii)from acetyl-CoA by means of a bifunctional dehydrogenase(adhE)with ac-etaldehyde and alcohol dehydrogenase activi-ties;adhE first converts acetyl-CoA to acetal-dehyde and then reduces the latter to ethanol. AdhE predominantly occurs in bacteria but has recently been identified in several protozoans, including vertebrate gut parasites such as Enta-moeba and Giardia(13,14).Adjacent to the adhE gene resides a second gene encoding only the AdhE C-terminal Fe-dependent alcohol de-hydrogenase domain.This gene product may form a multisubunit complex with AdhE,or it may function as an alternative alcohol dehydro-genase that is specific to certain growth condi-tions.C.parvum has a glycerol3-phosphate dehydrogenase similar to those of plants,fungi, and the kinetoplastid Trypanosoma,but(unlike trypanosomes)the parasite lacks an ortholog of glycerol kinase and thus this pathway does not yield glycerol production.In addition to themodular fatty acid synthase(Cp FAS1)andpolyketide synthase homolog(Cp PKS1), C.parvum possesses several fatty acyl–CoA syn-thases and a fatty acyl elongase that may partici-pate in fatty acid metabolism.Further,enzymesfor the metabolism of complex lipids(e.g.,glyc-erolipid and inositol phosphate)were identified inthe genome.Fatty acids are apparently not anenergy source,because enzymes of the fatty acidoxidative pathway are absent,with the exceptionof a3-hydroxyacyl-CoA dehydrogenase.C.parvum purine metabolism is greatlysimplified,retaining only an adenosine ki-nase and enzymes catalyzing conversionsof adenosine5Ј-monophosphate(AMP)toinosine,xanthosine,and guanosine5Ј-monophosphates(IMP,XMP,and GMP).Among these enzymes,IMP dehydrogenase(IMPDH)is phylogenetically related toε-proteobacterial IMPDH and is strikinglydifferent from its counterparts in both thehost and other apicomplexans(15).In con-trast to other apicomplexans such as Toxo-plasma gondii and P.falciparum,no geneencoding hypoxanthine-xanthineguaninephosphoribosyltransferase(HXGPRT)is de-tected,in contrast to a previous report on theactivity of this enzyme in C.parvum sporo-zoites(16).The absence of HXGPRT sug-gests that the parasite may rely solely on asingle enzyme system including IMPDH toproduce GMP from AMP.In contrast to otherapicomplexans,the parasite appears to relyon adenosine for purine salvage,a modelsupported by the identification of an adeno-sine transporter.Unlike other apicomplexansand many parasitic protists that can synthe-size pyrimidines de novo,C.parvum relies onpyrimidine salvage and retains the ability forinterconversions among uridine and cytidine5Ј-monophosphates(UMP and CMP),theirdeoxy forms(dUMP and dCMP),and dAMP,as well as their corresponding di-and triphos-phonucleotides.The parasite has also largelyshed the ability to synthesize amino acids denovo,although it retains the ability to convertselect amino acids,and instead appears torely on amino acid uptake from the host bymeans of a set of at least11amino acidtransporters(table S2).Most of the Cryptosporidium core pro-cesses involved in DNA replication,repair,transcription,and translation conform to thebasic eukaryotic blueprint(2).The transcrip-tional apparatus resembles Plasmodium interms of basal transcription machinery.How-ever,a striking numerical difference is seenin the complements of two RNA bindingdomains,Sm and RRM,between P.falcipa-rum(17and71domains,respectively)and C.parvum(9and51domains).This reductionresults in part from the loss of conservedproteins belonging to the spliceosomal ma-chinery,including all genes encoding Smdomain proteins belonging to the U6spliceo-somal particle,which suggests that this par-ticle activity is degenerate or entirely lost.This reduction in spliceosomal machinery isconsistent with the reduced number of pre-dicted introns in Cryptosporidium(5%)rela-tive to Plasmodium(Ͼ50%).In addition,keycomponents of the small RNA–mediatedposttranscriptional gene silencing system aremissing,such as the RNA-dependent RNApolymerase,Argonaute,and Dicer orthologs;hence,RNA interference–related technolo-gies are unlikely to be of much value intargeted disruption of genes in C.parvum.Cryptosporidium invasion of columnarbrush border epithelial cells has been de-scribed as“intracellular,but extracytoplas-mic,”as the parasite resides on the surface ofthe intestinal epithelium but lies underneaththe host cell membrane.This niche may al-low the parasite to evade immune surveil-lance but take advantage of solute transportacross the host microvillus membrane or theextensively convoluted parasitophorous vac-uole.Indeed,Cryptosporidium has numerousgenes(table S2)encoding families of putativesugar transporters(up to9genes)and aminoacid transporters(11genes).This is in starkcontrast to Plasmodium,which has fewersugar transporters and only one putative ami-no acid transporter(GenBank identificationnumber23612372).As a first step toward identification ofmulti–drug-resistant pumps,the genome se-quence was analyzed for all occurrences ofgenes encoding multitransmembrane proteins.Notable are a set of four paralogous proteinsthat belong to the sbmA family(table S2)thatare involved in the transport of peptide antibi-otics in bacteria.A putative ortholog of thePlasmodium chloroquine resistance–linkedgene Pf CRT(17)was also identified,althoughthe parasite does not possess a food vacuole likethe one seen in Plasmodium.Unlike Plasmodium,C.parvum does notpossess extensive subtelomeric clusters of anti-genically variant proteins(exemplified by thelarge families of var and rif/stevor genes)thatare involved in immune evasion.In contrast,more than20genes were identified that encodemucin-like proteins(18,19)having hallmarksof extensive Thr or Ser stretches suggestive ofglycosylation and signal peptide sequences sug-gesting secretion(table S2).One notable exam-ple is an11,700–amino acid protein with anuninterrupted stretch of308Thr residues(cgd3_720).Although large families of secretedproteins analogous to the Plasmodium multi-gene families were not found,several smallermultigene clusters were observed that encodepredicted secreted proteins,with no detectablesimilarity to proteins from other organisms(Fig.1,A and B).Within this group,at leastfour distinct families appear to have emergedthrough gene expansions specific to the Cryp-R E P O R T S SCIENCE VOL30416APRIL2004443o n O c t o b e r 7 , 2 0 0 9 w w w . s c i e n c e m a g . o r g D o w n l o a d e d f r o mtosporidium clade.These families —SKSR,MEDLE,WYLE,FGLN,and GGC —were named after well-conserved sequence motifs (table S2).Reverse transcription polymerase chain reaction (RT-PCR)expression analysis (20)of one cluster,a locus of seven adjacent CpLSP genes (Fig.1B),shows coexpression during the course of in vitro development (Fig.1C).An additional eight genes were identified that encode proteins having a periodic cysteine structure similar to the Cryptosporidium oocyst wall protein;these eight genes are similarly expressed during the onset of oocyst formation and likely participate in the formation of the coccidian rigid oocyst wall in both Cryptospo-ridium and Toxoplasma (21).Whereas the extracellular proteins described above are of apparent apicomplexan or lineage-specific in-vention,Cryptosporidium possesses many genesencodingsecretedproteinshavinglineage-specific multidomain architectures composed of animal-and bacterial-like extracellular adhe-sive domains (fig.S1).Lineage-specific expansions were ob-served for several proteases (table S2),in-cluding an aspartyl protease (six genes),a subtilisin-like protease,a cryptopain-like cys-teine protease (five genes),and a Plas-modium falcilysin-like (insulin degrading enzyme –like)protease (19genes).Nine of the Cryptosporidium falcilysin genes lack the Zn-chelating “HXXEH ”active site motif and are likely to be catalytically inactive copies that may have been reused for specific protein-protein interactions on the cell sur-face.In contrast to the Plasmodium falcilysin,the Cryptosporidium genes possess signal peptide sequences and are likely trafficked to a secretory pathway.The expansion of this family suggests either that the proteins have distinct cleavage specificities or that their diversity may be related to evasion of a host immune response.Completion of the C.parvum genome se-quence has highlighted the lack of conven-tional drug targets currently pursued for the control and treatment of other parasitic protists.On the basis of molecular and bio-chemical studies and drug screening of other apicomplexans,several putative Cryptospo-ridium metabolic pathways or enzymes have been erroneously proposed to be potential drug targets (22),including the apicoplast and its associated metabolic pathways,the shikimate pathway,the mannitol cycle,the electron transport chain,and HXGPRT.Nonetheless,complete genome sequence analysis identifies a number of classic and novel molecular candidates for drug explora-tion,including numerous plant-like and bacterial-like enzymes (tables S3and S4).Although the C.parvum genome lacks HXGPRT,a potent drug target in other api-complexans,it has only the single pathway dependent on IMPDH to convert AMP to GMP.The bacterial-type IMPDH may be a promising target because it differs substan-tially from that of eukaryotic enzymes (15).Because of the lack of de novo biosynthetic capacity for purines,pyrimidines,and amino acids,C.parvum relies solely on scavenge from the host via a series of transporters,which may be exploited for chemotherapy.C.parvum possesses a bacterial-type thymidine kinase,and the role of this enzyme in pyrim-idine metabolism and its drug target candida-cy should be pursued.The presence of an alternative oxidase,likely targeted to the remnant mitochondrion,gives promise to the study of salicylhydroxamic acid (SHAM),as-cofuranone,and their analogs as inhibitors of energy metabolism in the parasite (23).Cryptosporidium possesses at least 15“plant-like ”enzymes that are either absent in or highly divergent from those typically found in mammals (table S3).Within the glycolytic pathway,the plant-like PPi-PFK has been shown to be a potential target in other parasites including T.gondii ,and PEPCL and PGI ap-pear to be plant-type enzymes in C.parvum .Another example is a trehalose-6-phosphate synthase/phosphatase catalyzing trehalose bio-synthesis from glucose-6-phosphate and uridine diphosphate –glucose.Trehalose may serve as a sugar storage source or may function as an antidesiccant,antioxidant,or protein stability agent in oocysts,playing a role similar to that of mannitol in Eimeria oocysts (24).Orthologs of putative Eimeria mannitol synthesis enzymes were not found.However,two oxidoreductases (table S2)were identified in C.parvum ,one of which belongs to the same families as the plant mannose dehydrogenases (25)and the other to the plant cinnamyl alcohol dehydrogenases.In principle,these enzymes could synthesize protective polyol compounds,and the former enzyme could use host-derived mannose to syn-thesize mannitol.References and Notes1.D.G.Korich et al .,Appl.Environ.Microbiol.56,1423(1990).2.See supportingdata on Science Online.3.M.J.Gardner et al .,Nature 419,498(2002).4.A.T.Bankier et al .,Genome Res.13,1787(2003).5.J.C.Wootton,Comput.Chem.18,269(1994).Fig.1.(A )Schematic showing the chromosomal locations of clusters of potentially secreted proteins.Numbers of adjacent genes are indicated in paren-theses.Arrows indicate direc-tion of clusters containinguni-directional genes (encoded on the same strand);squares indi-cate clusters containingg enes encoded on both strands.Non-paralogous genes are indicated by solid gray squares or direc-tional triangles;SKSR (green triangles),FGLN (red trian-gles),and MEDLE (blue trian-gles)indicate three C.parvum –specific families of paralogous genes predominantly located at telomeres.Insl (yellow tri-angles)indicates an insulinase/falcilysin-like paralogous gene family.Cp LSP (white square)indicates the location of a clus-ter of adjacent large secreted proteins (table S2)that are cotranscriptionally regulated.Identified anchored telomeric repeat sequences are indicated by circles.(B )Schematic show-inga select locus containinga cluster of coexpressed large secreted proteins (Cp LSP).Genes and intergenic regions (regions between identified genes)are drawn to scale at the nucleotide level.The length of the intergenic re-gions is indicated above or be-low the locus.(C )Relative ex-pression levels of CpLSP (red lines)and,as a control,C.parvum Hedgehog-type HINT domain gene (blue line)duringin vitro development,as determined by semiquantitative RT-PCR usingg ene-specific primers correspondingto the seven adjacent g enes within the CpLSP locus as shown in (B).Expression levels from three independent time-course experiments are represented as the ratio of the expression of each gene to that of C.parvum 18S rRNA present in each of the infected samples (20).R E P O R T S16APRIL 2004VOL 304SCIENCE 444 o n O c t o b e r 7, 2009w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o m。

Remnant PbI2, an unforeseen necessity in high-efficiency hybrid perovskite-based solar cells?a)Duyen H. Cao, Constantinos C. Stoumpos, Christos D. Malliakas, Michael J. Katz, Omar K. Farha, Joseph T. Hupp, and Mercouri G. KanatzidisCitation: APL Materials 2, 091101 (2014); doi: 10.1063/1.4895038View online: /10.1063/1.4895038View Table of Contents: /content/aip/journal/aplmater/2/9?ver=pdfcovPublished by the AIP PublishingArticles you may be interested inParameters influencing the deposition of methylammonium lead halide iodide in hole conductor free perovskite-based solar cellsAPL Mat. 2, 081502 (2014); 10.1063/1.4885548Air stability of TiO2/PbS colloidal nanoparticle solar cells and its impact on power efficiencyAppl. Phys. Lett. 99, 063512 (2011); 10.1063/1.3617469High efficiency mesoporous titanium oxide PbS quantum dot solar cells at low temperatureAppl. Phys. Lett. 97, 043106 (2010); 10.1063/1.3459146Near-IR activity of hybrid solar cells: Enhancement of efficiency by dissociating excitons generated in PbS nanoparticlesAppl. Phys. Lett. 96, 073505 (2010); 10.1063/1.3292183Effects of molecular interface modification in hybrid organic-inorganic photovoltaic cellsJ. Appl. Phys. 101, 114503 (2007); 10.1063/1.2737977APL MATERIALS2,091101(2014)Remnant PbI2,an unforeseen necessity in high-efficiency hybrid perovskite-based solar cells?aDuyen H.Cao,1Constantinos C.Stoumpos,1Christos D.Malliakas,1Michael J.Katz,1Omar K.Farha,1,2Joseph T.Hupp,1,band Mercouri G.Kanatzidis1,b1Department of Chemistry,and Argonne-Northwestern Solar Energy Research(ANSER)Center,Northwestern University,2145Sheridan Road,Evanston,Illinois60208,USA2Department of Chemistry,Faculty of Science,King Abdulaziz University,Jeddah,Saudi Arabia(Received2May2014;accepted26August2014;published online18September2014)Perovskite-containing solar cells were fabricated in a two-step procedure in whichPbI2is deposited via spin-coating and subsequently converted to the CH3NH3PbI3perovskite by dipping in a solution of CH3NH3I.By varying the dipping time from5s to2h,we observe that the device performance shows an unexpectedly remark-able trend.At dipping times below15min the current density and voltage of thedevice are enhanced from10.1mA/cm2and933mV(5s)to15.1mA/cm2and1036mV(15min).However,upon further conversion,the current density decreases to9.7mA/cm2and846mV after2h.Based on X-ray diffraction data,we determinedthat remnant PbI2is always present in these devices.Work function and dark currentmeasurements showed that the remnant PbI2has a beneficial effect and acts as ablocking layer between the TiO2semiconductor and the perovskite itself reducingthe probability of back electron transfer(charge recombination).Furthermore,wefind that increased dipping time leads to an increase in the size of perovskite crys-tals at the perovskite-hole-transporting material interface.Overall,approximately15min dipping time(∼2%unconverted PbI2)is necessary for achieving optimaldevice efficiency.©2014Author(s).All article content,except where otherwisenoted,is licensed under a Creative Commons Attribution3.0Unported License.[/10.1063/1.4895038]With the global growth in energy demand and with compelling climate-related environmental concerns,alternatives to the use of non-renewable and noxious fossil fuels are needed.1One such alternative energy resource,and arguably the only legitimate long-term solution,is solar energy. Photovoltaic devices which are capable of converting the photonflux to electricity are one such device.2Over the last2years,halide hybrid perovskite-based solar cells with high efficiency have engendered enormous interest in the photovoltaic community.3,4Among the perovskite choices, methylammonium lead iodide(MAPbI3)has become the archetypal light absorber.Recently,how-ever,Sn-based perovskites have been successfully implemented in functional solar cells.5,6MAPbI3 is an attractive light absorber due to its extraordinary absorption coefficient of1.5×104cm−1 at550nm;7it would take roughly1μm of material to absorb99%of theflux at550nm.Further-more,with a band gap of1.55eV(800nm),assuming an external quantum efficiency of90%,a maximum current density of ca.23mA/cm2is attainable with MAPbI3.Recent reports have commented on the variability in device performance as a function of perovskite layer fabrication.8In our laboratory,we too have observed that seemingly identicalfilmsa Invited for the Perovskite Solar Cells special topic.b Authors to whom correspondence should be addressed.Electronic addresses:j-hupp@ and m-kanatzidis@2,091101-12166-532X/2014/2(9)/091101/7©Author(s)2014FIG.1.X-ray diffraction patterns of CH3NH3PbI3films with increasing dipping time(%composition of PbI2was determined by Rietveld analysis(see Sec.S3of the supplementary material for the Rietveld analysis details).have markedly different device performance.For example,when ourfilms of PbI2are exposed to MAI for several seconds(ca.60s),then a light brown coloredfilm is obtained rather than the black color commonly observed for bulk MAPbI3(see Sec.S2of the supplementary material for the optical band gap of bulk MAPbI3).23This brown color suggests only partial conversion to MAPbI3and yields solar cells exhibiting a J sc of13.4mA/cm2and a V oc of960mV;these values are significantly below the21.3mA/cm2and1000mV obtained by others.4Under the hypothesis that fully converted films will achieve optimal light harvesting efficiency,we increased the conversion time from seconds to2h.Unexpectedly,the2-h dipping device did not show an improved photovoltaic response(J sc =9.7mA/cm2,V oc=846mV)even though conversion to MAPbI3appeared to be complete.With the only obvious difference between these two devices being the dipping time,we hypothesized that the degree of conversion of PbI2to the MAPbI3perovskite is an important parameter in obtaining optimal device performance.We thus set out to understand the correlation between the method of fabrication of the MAPbI3layer,the precise chemical compositions,and both the physical and photo-physical properties of thefilm.We report here that remnant PbI2is crucial in forming a barrier layer to electron interception/recombination leading to optimized J sc and V oc in these hybrid perovskite-based solar cells.We constructed perovskite-containing devices using a two-step deposition method according to a reported procedure with some modifications.4(see Sec.S1of the supplementary material for the experimental details).23MAPbI3-containing photo-anodes were made by varying the dipping time of the PbI2-coated photo-anode in MAI solution.In order to minimize the effects from unforeseen variables,care was taken to ensure that allfilms were prepared in an identical manner.The composi-tions offinal MAPbI3-containingfilms were monitored by X-ray diffraction(XRD).Independently of the dipping times,only theβ-phase of the MAPbI3is formed(Figure1).9However,in addition to theβ-phase,allfilms also showed the presence of unconverted PbI2(Figure1,marked with*) which can be most easily observed via the(001)and(003)reflections at2θ=12.56◦and38.54◦respectively.As the dipping time is increased,the intensities of PbI2reflections decrease with a concomitant increase in the MAPbI3intensities.In addition to the decrease in peak intensities of PbI2,the peak width increases as the dipping time increases indicating that the size of the PbI2 crystallites is decreasing,as expected,and the converse is observed for the MAPbI3reflections.This observation suggests that the conversion process begins from the surface of the PbI2crystallites and proceeds toward the center where the crystallite domain size of the MAPbI3phase increases and that of PbI2diminishes.Interestingly,the remnant PbI2phase can be seen in the data of other reports, but has not been identified as a primary source of variability in cell performance.8,10 Considering that the perovskite is the primary light absorber within the device,we wantedto further investigate how the optical absorption of thefilm changes with increasing dipping timeFIG.2.Absorption spectra of CH3NH3PbI3films as a function of unconverted PbI2phase fraction.FIG.3.(a)J-V curves and(b)EQE of CH3NH3PbI3-based devices as a function of unconverted PbI2phase fraction.(Figure2).11,12The pure PbI2film shows a band gap of2.40eV,consistent with the yellow color of PbI2.As the PbI2film is gradually converted to the perovskite,the band gap is progressively shifted toward1.60eV.The deviation of MAPbI3’s band gap(1.60eV)from that of the bulk MAPbI3 material(1.55eV)could be explained by quantum confinement effects related with the sizes of TiO2and MAPbI3crystallites and their interfacial interaction.13,14Interestingly,we also noticed the presence of a second absorption in the light absorber layer,in which the gap gradually red shifts from1.90eV to1.50eV as the PbI2concentration is decreased from9.5%to0.3%(Figure2—blue arrow).Having established the chemical compositions and optical properties of the light absorberfilms, we proceeded to examine the photo-physical responses of the corresponding functional devices in order to determine how the remnant PbI2affects device performance.The pure PbI2based device remarkably achieved a0.4%efficiency with a J sc of2.1mA/cm2and a V oc of564mV (Figure3(a)).Upon progressive conversion of the PbI2layer to MAPbI3,we observe two different regions(Figure4,Table I).In thefirst region,the expected behavior is observed;as more PbI2is converted to MAPbI3,the trend is toward higher photovoltaic efficiency,due both to J sc and V oc, until1.7%PbI2is reached.The increase in J sc is attributable,at least in part,to increasing absorption of light by the perovskite.We speculate that progressive elimination of PbI2,present as a layer between TiO2and the perovskite,also leads to higher net yields for electron injection into TiO2and therefore,higher J values.For a sufficiently thick PbI2spacer layer,electron injection would occur instepwise fashion,i.e.,perovskite→PbI2→TiO2.Finally,the photovoltage increase is attributable toFIG.4.Summary of J-V data vs.PbI2concentration of CH3NH3PbI3-based devices(Region1:0to15min dipping time, Region2:15min to2h dipping time).TABLE I.Photovoltaic performance of CH3NH3PbI3-based devices as a function of unconverted PbI2fraction.Dipping time PbI2concentration a J sc(mA/cm2)V oc(V)Fill factor(%)Efficiency(%) 0s100% 2.10.564320.45s9.5%10.10.93352 4.960s7.2%13.40.96052 6.72min 5.3%14.00.964557.45min 3.7%14.70.995578.315min 1.7%15.1 1.036629.730min0.8%13.60.968648.51h0.4%12.40.938657.62h0.3%9.70.84668 5.5a Determined from the Rietveld analysis of X-ray diffraction data.the positive shift in TiO2’s quasi-Fermi level as the population of photo-injected electrons is higher with increased concentration of MAPbI3.The second region yields a notably different trend;surprisingly,below a concentration of2% PbI2,J sc,V oc,and ultimatelyηdecrease.Considering that the light-harvesting efficiency would increase when the remaining2%PbI2is converted to MAPbI3(albeit to only a small degree),then the remnant PbI2must have some other role.We posit that remnant PbI2serves to inhibit detrimental electron-transfer processes(Figure5).Two such processes are back electron transfer from TiO2to holes in the valence band of the perovskite(charge-recombination)or to the holes in the HOMO of the HTM(charge-interception).This retardation of electron interception/recombination observation is reminiscent of the behavior of atomic layer deposited Al2O3/ZrO2layers that have been employed in dye-sensitized solar cells.15–18It is conceivable that the conversion of PbI2to MAPbI3occurs from the solution interface toward the TiO2/PbI2interface and thus would leave sandwiched between TiO2and MAPbI3a blocking layer of PbI2that inhibits charge-interception/recombination.For this hypothesis to be correct,it is crucial that the conduction-band-edge energy(E cb)of the PbI2be higher than the E cb of the TiO2.19–21 The work function of PbI2was measured by ultraviolet photoelectron spectroscopy(UPS)and was observed to be at6.35eV vs.vacuum level,which is0.9eV lower than the valence-band-edge energy(E vb)of MAPbI3(see Sec.S7of the supplementary material23for the work function of PbI2);the E cb(4.05eV)was calculated by subtracting the work function from the band gap(2.30eV).solar cell.FIG.6.Dark current of CH3NH3PbI3-based devices as a function of unconverted PbI2phase fraction.The E cb of PbI2is0.26eV higher than the E cb of TiO2and thus PbI2satisfies the conditions of a charge-recombination/interception barrier layer.In order to probe the hypothesis that PbI2acts as a charge-interception barrier,dark current measurements,in which electronsflow from TiO2to the HOMO of the HTM,were made.Consistent with our hypothesis,Figure6illustrates that the onset of the dark current occurs at lower potentials as the PbI2concentration decreases.In the absence of other effects,the increasing dark current with increasing fraction of perovskite(and decreasing fraction of PbI2)should result in progressively lower open-circuit photovoltages.Instead,the photocurrent density and the open-circuit photovoltage bothincrease,at least until to PbI2fraction reaches1.7%.As discussed above,thinning of a PbI2-basedFIG.7.Cross-sectional SEM images of CH3NH3PbI3film with different dipping time.sandwich layer should lead to higher net injection yields,but excessive thinning would diminish the effectiveness of PbI2as a barrier layer for back electron transfer reactions.Given the surprising role of remnant PbI2in these devices,we further probed the two-step conversion process by using scanning-electron microscopy(SEM)(Figure7).Two domains of lead-containing materials(PbI2and MAPbI3)are present.Thefirst domain is sited within the mesoporous TiO2network(area1)while the second grows on top of the network(area2).Area2initially contains 200nm crystals.As the dipping time is increased,the crystals show marked changes in size and morphology.The formation of bigger perovskite crystals is likely the result of the thermodynamically driven Ostwald ripening process,i.e.,smaller perovskite crystals dissolves and re-deposits onto larger perovskite crystals.22The rate of charge-interception,as measured via dark current,is proportional to the contact area between the perovskite and the HTM.Thus,the eventual formation of large, high-aspect-ratio crystals,as shown in Figure7,may well lead to increases in contact area and thereby contributes to the dark-current in Figure6.Regardless,we found that the formation of large perovskite crystals greatly decreased our success rate in constructing high-functioning,non-shorting solar cells.In summary,residual PbI2appears to play an important role in boosting overall efficiencies for CH3NH3PbI3-containing photovoltaics.PbI2’s role appears to be that of a TiO2-supported blocking layer,thereby slowing rates of electron(TiO2)/hole(perovskite)recombination,as well as decreasing rates of electron interception by the hole-transporting material.Optimal performance for energy conversion is observed when ca.98%of the initially present PbI2has been converted to the perovskite. Conversion to this extent requires about15min.Pushing beyond98%(and beyond15min of reaction time)diminishes cell performance and diminishes the success rate in constructing non-shorting cells.The latter problem is evidently a consequence of conversion of small and more-or-less uniformly packed perovskite crystallites to larger,poorly packed crystallites of varying shape and size.Finally,the essential,but previously unrecognized,role played by remnant PbI2 provides an additional explanation for why cells prepared dissolving and then depositing pre-formed CH3NH3PbI3generally under-perform those prepared via the intermediacy of PbI2.We thank Prof.Tobin Marks for use of the solar simulator and EQE measurement system. Electron microscopy was done at the Electron Probe Instrumentation Center(EPIC)at Northwestern University.Ultraviolet Photoemission Spectroscopy was done at the Keck Interdisciplinary SurfaceScience facility(Keck-II)at Northwestern University.This research was supported as part of theANSER Center,an Energy Frontier Research Center funded by the U.S Department of Energy, Office of Science,Office of Basic Energy Sciences,under Award No.DE-SC0001059.1R.Monastersky,Nature(London)497(7447),13(2013).2H.J.Snaith,J.Phys.Chem.Lett.4(21),3623(2013).3M.M.Lee,J.Teuscher,T.Miyasaka,T.N.Murakami,and H.J.Snaith,Science338(6107),643(2012).4J.Burschka,N.Pellet,S.J.Moon,R.Humphry-Baker,P.Gao,M.K.Nazeeruddin,and M.Gratzel,Nature(London) 499(7458),316(2013).5F.Hao,C.C.Stoumpos,D.H.Cao,R.P.H.Chang,and M.G.Kanatzidis,Nat.Photonics8(6),489(2014);F.Hao,C.C. Stoumpos,R.P.H.Chang,and M.G.Kanatzidis,J.Am.Chem.Soc.136,8094–8099(2014).6N.K.Noel,S.D.Stranks,A.Abate,C.Wehrenfennig,S.Guarnera,A.Haghighirad,A.Sadhanala,G.E.Eperon,M.B. Johnston,A.M.Petrozza,L.M.Herz,and H.J.Snaith,Energy Environ.Sci.7,3061(2014).7H.S.Kim,C.R.Lee,J.H.Im,K.B.Lee,T.Moehl,A.Marchioro,S.J.Moon,R.Humphry-Baker,J.H.Yum,J.E.Moser, M.Gratzel,and N.G.Park,Sci.Rep.2,591(2012).8D.Y.Liu and T.L.Kelly,Nat.Photonics8(2),133(2014).9C.C.Stoumpos,C.D.Malliakas,and M.G.Kanatzidis,Inorg.Chem.52(15),9019(2013).10J.H.Noh,S.H.Im,J.H.Heo,T.N.Mandal,and S.I.Seok,Nano Lett.13(4),1764(2013).11Diffuse reflectance measurements of MAPbI3films were converted to absorption spectra using the Kubelka-Munk equation,α/S=(1-R)2/2R,where R is the percentage of reflected light,andαand S are the absorption and scattering coefficients, respectively.The band gap values are the energy value at the intersection point of the absorption spectrum’s tangent line and the energy axis.12L.F.Gate,Appl.Opt.13(2),236(1974).13O.V oskoboynikov,C.P.Lee,and I.Tretyak,Phys.Rev.B63(16),165306(2001).14X.X.Xue,W.Ji,Z.Mao,H.J.Mao,Y.Wang,X.Wang,W.D.Ruan,B.Zhao,and J.R.Lombardi,J.Phys.Chem.C 116(15),8792(2012).15E.Palomares,J.N.Clifford,S.A.Haque,T.Lutz,and J.R.Durrant,J.Am.Chem.Soc.125(2),475(2003).16C.Prasittichai,J.R.Avila,O.K.Farha,and J.T.Hupp,J.Am.Chem.Soc.135(44),16328(2013).17A.K.Chandiran,M.K.Nazeeruddin,and M.Gratzel,Adv.Funct.Mater.24(11),1615(2014).18M.J.Katz,M.J.D.Vermeer,O.K.Farha,M.J.Pellin,and J.T.Hupp,Langmuir29(2),806(2013).19M.J.DeVries,M.J.Pellin,and J.T.Hupp,Langmuir26(11),9082(2010).20C.Prasittichai and J.T.Hupp,J.Phys.Chem.Lett.1(10),1611(2010).21F.Fabregat-Santiago,J.Garcia-Canadas,E.Palomares,J.N.Clifford,S.A.Haque,J.R.Durrant,G.Garcia-Belmonte, and J.Bisquert,J.Appl.Phys.96(11),6903(2004).22Alan D.McNaught and Andrew Wilkinson,IUPAC Compendium of Chemical Terminology(Blackwell Scientific Publica-tions,Oxford,1997).23See supplementary material at /10.1063/1.4895038for experimental details,absorption spectrum of bulk CH3NH3PbI3,fraction,size,absorption spectrum,work function of unconverted PbI2,and average photovoltaic perfor-mance.。

Crystallization of Amino Acids on Self-AssembledMonolayers of Rigid Thiols on GoldAlfred Y.Lee,†,‡Abraham Ulman,†,§and Allan S.Myerson*,‡Department of Chemical and Environmental Engineering,Illinois Institute of Technology, Chicago,Illinois60616,and Department of Chemical Engineering,Chemistry and Material Science,and the NSF MRSEC for Polymers at Engineered Interfaces,Polytechnic University,Brooklyn,New York11201Received March6,2002.In Final Form:May3,2002Self-assembled monolayers(SAMs)of rigid biphenyl thiols are employed as heterogeneous nucleants for the crystallization of L-alanine and DL-valine.Powder X-ray diffraction and interfacial angle measurements reveal that the L-alanine crystallographic planes corresponding to nucleation are{200}, {020},and{011}on SAMs of4′-hydroxy-(4-mercaptobiphenyl),4′-methyl-(4-mercaptobiphenyl),and4-(4-mercaptophenyl)pyridine on gold(111)surfaces,respectively.In the case of DL-valine,monolayer surfaces that act as hydrogen bond acceptors(e.g.,4′-hydroxy-(4-mercaptobiphenyl)and4-(4-mercaptophenyl)-pyridine)induce the racemic crystal to nucleate from the{020}plane whereas the nucleating plane for the4′-methyl-(4-mercaptobiphenyl)surface is the fast-growing{100}face.The observation of crystal nucleation and orientation can be attributed to the strong interfacial interactions,in particular,hydrogen bonding,between the surface functionalities of the monolayer film and the individual molecules of the crystallizing phase.Molecular modeling studies are also undertaken to examine the molecular recognition process across the interface between the surfactant monolayer and the crystallographic planes.Similar to binding studies of solvents and impurities on crystal habit surfaces,binding energies between SAMs and particular amino acid crystal faces are calculated and the results are in good agreement with the observed nucleation planes of the amino acids.In addition to L-alanine and DL-valine,the interaction of SAMs and mixed SAMs of rigid thiols on the morphology of R-glycine is examined(Kang,J.F.;Zaccaro, J.;Ulman,A.;Myerson,ngmuir2000,16,3791),and similarly the calculations are in good agreement. These results suggest that binding energy calculations can be a valid method to screen self-assembled monolayers as potential templates for nucleation and growth of organic and inorganic crystals.I.IntroductionCrystallization from solution is a two-step process: nucleation,the birth of a crystal,and crystal growth,the growth of the crystal to larger sizes.2In this process, prenucleation aggregates(or clusters)are formed by individual molecules,which become stable nuclei,upon reaching a critical size,and further grow into macroscopic crystals.Homogeneous nucleation is very rare and re-quires high supersaturation to surmount the activation barrier,∆G crit.However,for a fixed supersaturation the activation barrier can be lowered by decreasing the surface energy of the aggregate,for instance,by introducing a foreign surface or substance.3This foreign surface(or substance)includes“tailor-made”additives,4impurities,5 organic single crystals,6Langmuir monolayers7floating at the air-water interface,and self-assembled monolayers (SAMs)immersed in solution.1Tailor-made additives or auxiliaries are designer impurities that have one part which resembles the crystallizing species and another part that is chemically or structurally different from the solute molecule.4,8These additives disrupt the bonding sequence in the crystals,thereby lowering the growth rate of the affected faces as evident in the case of L-alanine where hydrophobic amino acids such as L-leucine and L-valine inhibited the development of specific crystal faces,while in the presence of hydrophilic amino acids the crystal morphology did not change.9In addition to being habit modifiers,these molecular additives can also control polymorphism,where the impurities inhibit the growth of one polymorph and,in turn,promote the growth of the other polymorph.10Nucleation promoters such as organic single crystals and self-assembled monolayers have also been used to control polymorph selectivity,based on geometric match-ing between the molecular clusters and the ledges of the crystal substrates11and interfacial hydrogen bonding between the monolayer film and solute clusters,12respec-*To whom correspondence should be addressed.Phone:312 5677010.Fax:3125677018.E-mail:myerson@.†Polytechnic University.‡Illinois Institute of Technology.§NSF MRSEC for Polymers at Engineered Interfaces.(1)Kang,J.F.;Zaccaro,J.;Ulman,A.;Myerson,ngmuir2000, 16,3791.(2)(a)Myerson,A.S.Handbook of Industrial Crystallization,2nd ed.;Butterworth-Heinemann:Boston,2002.(b)Myerson,A.S.Mo-lecular Modeling Applications in Crystallization;Cambridge Uni-versity Press:New York,1999.(c)Mullin,J.W.Crystallization,4th ed.;Butterworth-Heinemann:Boston,2001.(3)Turnbull,D.J.Chem.Phys.1949,18,198.(b)Fletcher,N.H.J. Chem.Phys.1963,38,237.(4)Weissbuch,I.;Lahav,M.;Leiserowitz,L.In Molecular Modeling Applications in Crystallization;Myerson, A.S.,Ed.;Cambridge University Press:New York,1999;p166.(5)Meenan,P.A.;Anderson,S.R.;Klug,D.L.In Handbook of Industrial Crystallization,2nd ed.;Myerson,A.S.,Ed.;Butterworth Heinemann:Boston,2002;p67.(6)Carter,P.W.;Ward,M.D.J.Am.Chem.Soc.1993,115,11521.(7)(a)Rapaport,H.;Kuzmenko,I.;Berfeld,M.;Kjaer,K.;Als-Nielsen, J.;Popovitz-Biro,R.;Weissbuch,I.;Lahav,M.;Leiserowitz,L.J.Phys. Chem.B2000,104,1399.(b)Frostman,L.M.;Ward,ngmuir 1997,13,330.(8)(a)Berkovitch-Yellin,Z.;Ariel,S.;Leiserowitz,L.J.Am.Chem. Soc.1985,105,765.(b)Addadi,L.;Weinstein,S.;Gate,E.;Weissbuch,I.;Lahav,M.J.Am.Chem.Soc.1982,104,4610.(9)Li,L.;Lechuga-Ballesteros,D.;Szkudlarek,B.A.;Rodriguez-Hornedo,N.J.Colloid Interface Sci.1994,168,8.(10)(a)Weissbuch,I.;Lahav,M.;Leiserowitz,L.Adv.Mater.1994, 6,952.(b)Davey,R.J.;Blagden,N.;Potts,G.D.;Docherty,R.J.Am. Chem.Soc.1997,119,1767.5886Langmuir2002,18,5886-589810.1021/la025704w CCC:$22.00©2002American Chemical SocietyPublished on Web06/22/2002tively.Similar to Langmuir monolayers,self-assembled monolayers can be used as an interface across which stereochemical matching13and hydrogen bonding14in-teraction can transfer order and symmetry from the monolayer surface to a growing crystal.However,SAMs and mixed SAMs15lack the mobility of molecules at an air-water interface and hence the possibility to adjust lateral positions to match a face of a nucleating crystal. This is clearly evident in the case of the SAMs of rigid biphenyl thiols,where even conformational adjustment is not possible.Recently,SAMs of4-mercaptobiphenyl have been shown to be more superior to those of al-kanethiolates and are stable model surfaces.16Further-more,the ability to engineer surface functionalities at the molecular level makes SAMs of rigid thiols very attractiveas templates for heterogeneous nucleation. Organosilane monolayer films have been used to promote nucleation and growth of calcium oxalate mono-hydrate crystals17and have been employed in“biomimetic”synthesis as observed in the oriented growth of CaCO318 and iron hydroxide crystals.19Functionalized SAMs of alkanethiols have also been shown to control the oriented growth of CaCO3.20This was also evident in the hetero-geneous nucleation and growth of malonic acid crystals21 on alkanethiolate SAMs on gold where the monolayer composition strongly influenced the orientation of the malonic acid crystals.Additionally,functionalized alkane-thiolate SAMs have enhanced the growth of protein crystals.22More recently,SAMs and mixed SAMs of rigid thiols served as templates.1It was observed that glycine nucleated in the R-form independent of the hydroxyl and pyridine surface concentration and the morphology of the glycine crystal was very sensitive to the OH and pyridine site densities.Self-assembled monolayers on solid surfaces offer many advantages for enhanced crystal nucleation.In this work, SAMs of rigid thiols on gold are employed to investigate the effects of interfacial molecular recognition on nucle-ation and growth of L-alanine and DL-valine crystals.In addition,molecular modeling techniques are employed to examine the affinity between monolayer surfaces and particular amino acid crystal faces and to gain a better understanding of the molecular recognition events oc-curring.The modeling techniques employed are similar to studies of solvent and additive interactions on crystal habit23but have never been applied to organic monolayer films as templates for nucleation.II.Experimental SectionMaterials.Anhydrous ethanol was obtained from Pharmco (Brookfield,CT).L-Alanine(CH3CH(NH2)CO2H),and DL-valine ((CH3)2CHCH(NH2)CO2H)were purchased from Aldrich and used without further purification.Distilled water purified with a Milli-Q water system(Millipore)was used.Details of the synthesis of the4′-substituted4-mercaptobiphenyl(see Figure1)are described elsewhere.24Gold Substrate and Monolayer Preparation.Glass slides were cleaned in ethanol in an ultrasonic bath at40°C for10min. The slides were next treated in a plasma chamber at an argon pressure of0.1Torr for30min.Afterward,they were mounted in the vacuum evaporator(Key High Vacuum)on a substrate holder,approximately15cm above the gold cluster.The slides were baked overnight in a vacuum(10-7Torr)at300°C.Gold (purity>99.99%)was evaporated at a rate of3-5Å/s until the film thickness reached1000Å;the evaporation rate and film thickness were monitored with a quartz crystal microbalance (TM100model from Maxtek Inc.).The gold substrates were annealed in a vacuum at300°C for18h.After cooling to room temperature,the chamber was filled with high-purity nitrogen and the gold slides were either placed into the adsorbing solution right after the ellipsometric measurement was performed or stored in a vacuum desiccator for later use.25Atomic force microscopy(AFM)studies24revealed terraces of Au(111)with typical crystalline sizes of0.5-1µm2.Monolayers were formed by overnight(∼18h)immersion of clean substrates in10µm ethanol solutions of the thiols.The substrates were removed from the solution,rinsed with copious amounts of absolute ethanol to remove unbound thiols,and blown dry with a jet of nitrogen. Contact angle measurements,IR spectroscopy,and ellipsometry showed that after1h,90%or more of the SAMs are formed.26 Thus,to ensure equilibrium SAMs,the gold substrates were left overnight in the dipping solution.Crystal Growth.Nucleation and growth experiments were carried out in Quartex jars(1oz.)at25°C.Supersaturated solutions(25%)of L-alanine and DL-valine were obtained by dissolving 4.58g and 1.95g in22.0g of Millipore water, respectively.The solutions were heated to65°C for90min in an ultrasonic bath to obtain complete dissolution.The solutions were cooled to room temperature for90min before the SAMs were carefully introduced and aligned vertically to the wall. Macrocrystals of L-alanine and DL-valine nucleated at the surfaces(11)(a)Bonafede,S.J.;Ward,M.D.J.Am.Chem.Soc.1995,117, 7853.(b)Mitchell,C.A.;Yu,L.;Ward,M.D.J.Am.Chem.Soc.2001, 123,10830.(12)Carter,P.W.;Ward,M.D.J.Am.Chem.Soc.1994,116,769.(13)(a)Landau,E.M.;Levanon,M.;Leiserowitz,L.;Lahav,M.;Sagiv, J.Nature1985,318,353.(b)Weissbuch,I.;Berfeld,M.;Bouwman,W.; Kjaer,K.;Als,J.;Lahav,M.;Leiserowitz,L.J.Am.Chem.Soc.1997, 119,933.(14)Weissbuch,I.;Popvitz,R.;Lahav,M.;Leiserowitz,L.Acta Crystallogr.1995,B51,115.(15)For a review on SAMs of thiols on gold see:(a)Ulman,A.An Introduction to Ultrathin Organic Films:From Langmuir-Blodgett to Self-Assembly;Academic Press:Boston,1991.(b)Ulman,A.Chem. Rev.1996,96,1533.(16)(a)Kang,J.F.;Ulman,A.;Liao,S.;Jordan,R.J.Am.Chem.Soc. 1998,120,9662.(b)Kang,J.F.;Jordan,R.;Ulman,ngmuir1998,14,3983.(17)Campbell,A.A.;Fryxell,G.E.;Graff,G.L.;Rieke,P.C.; Tarasevich,B.J.Scanning Microsc.1993,7(1),423.(18)Archibald,D.D.;Qadri,S.B.;Gaber,ngmuir1996,12, 538.(19)Tarasevich,B.J.;Rieke,P.C.;Liu,J.Chem.Mater.1996,8,292.(20)Aizenberg,J.;Black,A.J.;Whitesides,G.M.J.Am.Chem.Soc. 1999,121,4500.(21)Frostman,L.M.;Bader,M.M.;Ward,ngmuir1994, 10,576.(22)Ji,D.;Arnold,C.M.;Graupe,M.;Beadle,E.;Dunn,R.V.;Phan, M.N.;Villazana,R.J.;Benson,R.;Colorado,R.,Jr.;Lee,T.R.;Friedman, J.M.J.Cryst.Growth2000,218,390.(23)(a)Docherty,R.;Meenan,P.In Molecular Modeling Applications in Crystallization;Myerson,A.S.,Ed.;Cambridge University Press: New York,1999;p106.(b)Myerson,A.S.;Jang,S.M.J.Cryst.Growth 1995,156,459.(c)Walker,E.M.;Roberts,K.J.;Maginn,ngmuir 1998,14,5620.(d)Evans,J.;Lee,A.Y.;Myerson,A.S.In Crystallization and Solidification Properties of Lipids;Widlak,N.,Hartel,R.W.,Narine, S.,Eds.;AOCS Press:Champaign,IL,2001;p17.(24)Kang,J.F.;Ulman,A.;Liao,S.;Jordan,R.;Yang,G.;Liu,G. Langmuir2001,17,95.(25)(a)Jordan,R.;Ulman,A.J.Am.Chem.Soc.1998,120,243.(b) Jordan,R.;Ulman,A.;Kang,J.F.;Rafailovich,M.;Sokolov,J.J.Am. Chem.Soc.1999,121,1016.(26)Ulman,A.Acc.Chem.Res.2001,34,855.Figure1.Rigid4′-substituted4-mercaptobiphenyls.Crystallization of Amino Acids on Thiol SAMs Langmuir,Vol.18,No.15,20025887and near the edge of the substrates.Only crystals having visible SAM area around them were considered,and the rest were discarded.The chosen crystals attached to the substrates were removed from the solution and stored in a vacuum desiccator for later analysis.Due to the strong adhesion of the crystal face to the SAM surface,gold marks were often observed on the crystal face that nucleated on the SAM surface.Characterization.A Rudolph Research AutoEL ellipsometer was used to measure the thickness of the monolayer surface.The He -Ne laser (632.8nm)light fell at 70°on the sample and reflected into the analyzer.Data were taken over five to seven spots on each sample.The measured thickness of the SAMs of biphenyl thiols ranged from 12to 14Å,assuming a refractive index of 1.462for all films.Powder X-ray diffraction patterns of crystalline L -alanine and DL -valine were obtained with a Rigaku Miniflex diffractometer with Cu K R radiation (λ)1.5418Å).All samples were manually ground into fine powder and packed in glass slides for analysis.Data were collected from 5°to 50°with a step size of 0.1°.Crystal habits of L -alanine and DL -valine were indexed by measuring the interfacial angles using a two-circle optical goniometer.All possible measured interfacial angles were compared with the theoretical values derived from the unit cell parameters of L -alanine and DL -valine crystals.27,28III.Modeling SectionIII.1.General.All of the binding energy calculations,including molecular mechanics and dynamics simulations,are carried out with the program Cerius 2.The overall methodology and procedures are summarized in Figure 2.The crystal structures of each amino acid are obtained from the Cambridge Crystallographic Database (ref codes GLYCIN17,LALNIN12,and VALIDL for R -glycine,L -alanine,and DL -valine,respectively).To accurately predict the crystal morphology,molecular mechanics simulations using a suitable potential function (or force field)are performed.In this work,molecular simulations are carried out using the DREIDING 2.21force field.29The van der Waals forces are approximated with the Lennard-Jones 12-6expression,and hydrogen bonding energy is modeled using a Lennard-Jones-like 12-10expression.The Ewald summation technique is employed for the summation of long-range van der Waals and electrostatic interactions under the periodic boundary conditions,and the charge distribution within the molecule is calculated using the Gasteiger method.30ttice Energy Calculation.The lattice energy E lat,also known as the cohesive or crystal binding energy,is calculated by summing all the atom -atom interactions between a central molecule and all the surrounding molecules in the crystal.If the central molecule and the n surrounding molecules each have n ′atoms,thenwhere V kij is the interaction between atom i in the central molecule and atom j in the k th surrounding parison to the “experimental”lattice energy,V exp ,allows us to assess the accuracy of the intermolecular interactions between the molecules by the defined po-tential function.where the term 2RT represents a compensation factor for the difference between the vibrational contribution to the crystal enthalpy and gas-phase enthalpy 31and ∆H sub is the experimental sublimation energy.III.3.Morphological Predictions.The morphology of each amino acid crystal is predicted using the attach-ment energy (AE)32calculation and the Bravais -Friedel -Donnay -Harker (BFDH)law.33The habit or shape of the crystal depends on the growth rate of the faces present.Faces that are slow growing have the greatest morpho-logical importance,and conversely,faces that are fast growing have the least morphological importance and are the smallest faces on the grown crystal.The simplest morphological simulation is the BFDH law which assumes that the linear growth rate of a given crystal face is inversely proportional to the corresponding interplanar distance after taking into account the extinction conditions of the crystal space group.The attachment energy of a crystal face is the difference between the crystal energy and the slice energy.Hartman and Bennema 32found that the relative growth rate of a face is directly proportional to the attachment energy and as a result,the more negative the attachment energy (or more energy released)for a particular face,the less prominent that face is on the crystal.Conversely,faces with the lowest attachment energies are the slowest growing faces and thus have the greatest morphological importance .III.4.Molecular Modeling of SAMs of 4-Mercapto-biphenyls on a Au(111)Surface.Molecular dynamics (MD)simulations are useful techniques in gaining insights on the structural and dynamical properties of self-assembled monolayers.In contrast to molecular mechan-ics,molecular dynamics computes the forces and moves the atom in response to forces,while molecular mechanics computes the forces on the atoms and changes their position to minimize the interaction energy.Recently,MD simulations have been used to investigate the packing order and orientation of rigid 4-mercaptobiphenyl thiol monolayers on gold surfaces.Results show that hydrogen-terminated biphenylmercaptan packs in the herringbone conformation 34and suggest average tilt angles of 8°.(27)Simpson,H.J.;Marsh,R.E.Acta Crystallogr .1966,20,550.(28)Mallikarjunan,M.;Rao,S.T.Acta Crystallogr .1969,B25,296.(29)Mayo,S.L.;Olafson,B.D.;Goddard,W.A.,III J.Phys.Chem .1990,94,8897.(30)Gasteiger,J.;Marsili,M.Tetrahedron 1980,36,3219.(31)Williams,D.E.J.Phys.Chem .1966,45,3370.(32)(a)Hartman,P.;Bennema,P.J.Cryst.Growth 1980,49,145.(33)(a)Bravais,A.Etudes Crystallographiques ;Gauthier-Villars:Paris,1866.(b)Friedel,M.G.Bulletin de la Societe Francaise de Mineralogie 1907,30,326.(c)Donnay,J.D.;Harker,D.Am.Mineral.1937,22,446.Figure 2.Overall scheme showing the computational meth-odology adopted when calculating the binding energy betweenthe crystallographic plane and the monolayer surface.Elat)∑k )1n ∑i )1n ′∑j )1n ′V kij (1)V exp )-∆H sub -2RT(2)5888Langmuir,Vol.18,No.15,2002Lee et al.Based on this work,molecular mechanics simulations are performed for hydroxy-and methyl-terminated 4-mer-captobiphenyl along with 4-(4-mercaptophenyl)pyridine for binding studies with different crystallographic planes.In the periodic model,each unit cell contains four biphenyl molecules and the geometric parameters are a )10.02Å,b )42.25Å,c )10.11Åand R )138.3°, )119.9°,γ)95.7°.The length in the y -direction is set to ∼42Åto ensure two-dimensional periodicity.Also,the gold atoms are arranged in a hexagonal lattice along the XY plane with a nearest neighbor atom of 2.88Å,and the biphenyl occupied a ( 3× 3)R30°Au(111)lattice.To simulate different 4′-substituted 4-mercaptobiphenyls,minimiza-tion was carried out by fixing the biphenyl moiety and varying the substituents at the 4′-position.As a result,the simulated models yielded uniform ordered SAMs of 4′-substituted 4-mercaptobiphenyls and 4-(4-mercapto-phenyl)pyridine with identical packing structure and dynamics to those of a hydrogen-terminated monolayer of biphenylmercaptan (Figure 3).However,this is not true experimentally since adsorption of different 4′-substituted 4-mercaptobiphenyls on gold surfaces results in different monolayer structures and thus one of the main assump-tions made in this work.III.5.Binding of Crystal Habit Faces to SAMs of 4-Mercaptobiphenyls on a Au(111)Surface.Based on BFDH and attachment energy morphology prediction,crystal habit faces with the highest morphological im-portance are chosen for binding studies.The crystal surfaces of interest are cleaved and extended to a 3×3unit cell and partially fixed,allowing flexibility in the tail atoms of the amino acid molecules and a more accurate representation of the effects of SAMs of rigid thiols on the crystallographic plane in the calculation of binding energies.The crystal surface is then docked onto a 3×1×3partially fixed nonperiodic monolayer surface,and the conjugate gradient energy minimization technique is performed.Next,the crystal surface is moved to another site on the monolayer surface and the minimization calculations are again performed.This process was repeated 15-20times to obtain the global minimum.For each monolayer surface,numerous calculations are carried out with different crystallographic planes of each amino acid.The binding energy (φBE )of each crystallographic surface with the monolayer surface iswhere φIE is the minimum interaction energy of the monolayer and crystal surfaces,φM is the minimum energy of the monolayer surface in the absence of the crystal face but in the same conformation as it adopts on the surface,and φS is the minimum energy of the crystal surface with no monolayer surface present and in the same molecular conformation in which it docks on the surface.Negative values of binding energies indicate preferential binding of the crystallographic surfaces with SAMs of 4-mercaptobiphenyl.In cases where the binding energy is positive,there is a less likely chance that the particular crystal face will interact and nucleate on the monolayer surface.Thus,using this approach it is possible to screen self-assembled monolayers as possible templates for nucleation and growth of crystals.IV.Results and DiscussionIV.1.Crystallization of Amino Acids on SAMs on Gold.L -Alanine crystallizes from water in the ortho-rhombic space group P 21212(a )6.025Å,b )12.324Å,and c )5.783Å),27and the morphology of the crystals is bipyramidal,dominated by the {020},{120},{110},and {011}growth forms,35as shown in Figure 4.The crystal grown in aqueous solution is indexed by comparing the interfacial angles measured by optical goniometry and theoretical values based on the unit cell of L -alanine.Powder X-ray diffraction patterns (Figure 5)and inter-facial angle measurements reveal that L -alanine crystals nucleating on SAM surfaces crystallize in the ortho-rhombic space group with similar unit cell dimensions.However,functionalized SAMs induce the formation of L -alanine crystals in different crystallographic directions.L -Alanine crystals display the normal bipyramidal habit but are randomly oriented with the different surfaces.In methyl-terminated SAMs,L -alanine selectively nucle-ated on the {020}plane on the surface (Figure 6),whereas in 100%OH SAM surfaces,L -alanine nucleated on an unobserved {200}side face.The crystal exhibits a similar morphology as observed in aqueous solution with an appearance of a {200}face adjacent to the {110}planes (Figure 6).In both cases,the area of each crystal face is substantially larger than those of the other faces on the crystal.The SAM surfaces almost act as an additive or impurity molecule specifically interacting with the crystal face and consequently reducing the relative growth rate and modifying the habit.Crystallization of L -alanine on 4-(4-mercaptophenyl)pyridine surfaces resulted in the {011}face as the plane corresponding to nucleation (Figure 6).The preferential interaction of the monolayer with the {011}face can be attributed to hydrogen bonding at the crystal -monolayer interface.Unlike the other two sur-faces where they can serve as both hydrogen bond donors and acceptors (4′-hydroxy-4-mercaptobiphenyl)or solely as H-bond donors (4′-methyl-4-mercaptobiphenyl),the pyridine electron pair at the surface only serve as hydrogen bond acceptors.The binding of the pyridine surface and the {011}plane can be explained by the amino and methyl groups protruding out perpendicular to the plane (Figure 7)and forming N -H ‚‚‚N and C -H ‚‚‚N hydrogen bonds with the SAM surface,respectively.In contrast,the 100%(34)Ulman,A.;Kang,J.F.;Shnidman,Y.;Liao,S.;Jordan,R.;Choi,G.Y.;Zaccaro,J.;Myerson,A.S.;Rafailovich,M.;Sokolov,J.;Flesicher,C.Rev.Mol.Biotech .2000,74,175.(35)Lehmann,M.S.;Koetzle,T.F.;Hamilton,W.C.J.Am.Chem.Soc .1972.101,2657.Figure 3.Snapshots of (a)4′-methyl-4-mercaptobiphenyl,(b)4′-hydroxy-4-mercaptobiphenyl,(c)4-(4-mercaptophenyl)pyri-dine,and (d)mixed SAMs of 4′-methyl-4-mercaptobiphenyl and 4′-hydroxy-4-mercaptobiphenyl (top view).φBE )φIE -(φM +φS )(3)Crystallization of Amino Acids on Thiol SAMs Langmuir,Vol.18,No.15,20025889CH 3and 100%OH SAM surfaces do not interact as strongly with the hydrogen bond donating plane.In a similar manner,the appearance of an unobserved {200}face of L -alanine grown in aqueous solution on [Au]-S -C 6H 4-C 6H 4-OH can be attributed to hydrogen bonds forming between the two surfaces.The {200}surface contains alternating methyl (CH 3)and carboxylic groups (COO -)that form N -H ‚‚‚O and O ‚‚‚H -O with the hydroxide group of the monolayer film (Figure 7),ideal for binding with surfaces that can serve as both hydrogen bond donors and acceptors.As a result,the preferential interaction leads to the stabilization and appearance of the {200}face.The oriented nucleation of L -alanine crystals on func-tionalized SAMs arises due to the different molecular structures of each crystal face.Similar to the adsorption of additive onto a crystal face,the interaction (or binding)with the monolayer surface depends on the functional group that each crystal face possesses.As a result of preferential interactions with specific crystal faces,in-terfacial molecular recognition directs nucleation and subsequently influences the crystal growth.In addition to L -alanine,SAMs of rigid thiols are employed to investigate the possibility of inhibiting the racemic crystal and inducing the formation of one of its enantiomers.The powder X-ray diffraction pattern (Figure 8)reveals that DL -valine nucleates in the monoclinic form independent of the hydroxyl,methyl,or pyridine surface concentration and that there was no trace of conglomer-ates.DL -Valine crystallizes in the monoclinic space group P 21/c with a unit cell of dimensions a )5.21Å,b )22.10Å,c )5.41Å,and )109.2°.28Although the structural literature reports three separate space group assignments,Leiserowitz and co-workers 36have shown that two of the three space groups (P 21and P 1)are highly improbable for racemic crystals.Interfacial angle measurements and powder X-ray diffraction undertaken in this work agreed much better with the theoretical values and simulated pattern based on the unit cell of the monoclinic space group(36)Wolf,S.G.;Berkovitch-Yellin,Z.;Lahav,M.;Leiserowitz,L.Mol.Cryst.Liq.Cryst .1990,186,3.Figure 4.Crystallographic image (a)and morphology (b)of L -alanine crystal grown from aqueoussolution.Figure 5.X-ray diffractograms of L -alanine nucleated on functionalized SAMs,compared with L -alanine crystallized from aqueous solution (bottom).Indices of the crystallographic planes corresponding to the diffraction intensities of major peaks are indicated at the top.5890Langmuir,Vol.18,No.15,2002Lee et al.。

1. 产生X 射线需具备什么条件？答：实验证实:在高真空中，凡高速运动的电子碰到任何障碍物时，均能产生X 射线，对于其他带电的基本粒子也有类似现象发生。

电子式X 射线管中产生X 射线的条件可归纳为：1，以某种方式得到一定量的自由电子；2，在高真空中，在高压电场的作用下迫使这些电子作定向高速运动；3，在电子运动路径上设障碍物以急剧改变电子的运动速度。

2. 计算当管电压为50 kv 时，电子在与靶碰撞时的速度与动能以及所发射的连续谱的短波限和光子的最大动能解已知条件：U=50kv 电子静止质量：m 0=9.1×10-31kg 光速：c=2.998×108m/s 电子电量：e=1.602×10-19C 普朗克常数：h=6.626×10-34J.s 电子从阴极飞出到达靶的过程中所获得的总动能为 E=eU=1.602×10-19C ×50kv=8.01×10-18kJ 由于E=1/2m 0v 02 所以电子与靶碰撞时的速度为 v 0=(2E/m 0)1/2=4.2×106m/s所发射连续谱的短波限λ0的大小仅取决于加速电压λ0（Å）＝12400/v(伏) ＝0.248Å3. 辐射出来的光子的最大动能为 E 0＝h ʋ0＝hc/λ0＝1.99×10-15J 连续谱是怎样产生的？其短波限VeVhc 201024.1⨯==λ与某物质的吸收限kkk V eVhc 21024.1⨯==λ有何不同（V 和V K 以kv 为单位）？答 当ⅹ射线管两极间加高压时，大量电子在高压电场的作用下，以极高的速度向阳极轰击，由于阳极的阻碍作用，电子将产生极大的负加速度。

根据经典物理学的理论，一个带负电荷的电子作加速运动时，电子周围的电磁场将发生急剧变化，此时必然要产生一个电磁波，或至少一个电磁脉冲。

由于极大数量的电子射到阳极上的时间和条件不可能相同，因而得到的电磁波将具有连续的各种波长，形成连续ⅹ射线谱。

中国天⽂学会天⽂学名词审定委员会第1-6批天⽂学名词的推荐译名The 1st - 6th Drafts for the Chinese-Translation of Astronomical Terms recommanded byThe Astronomical Terminology Committee of the CASabsolute stability 绝对稳定性absorbing dust mass 致吸尘物质absorption trough 吸收槽abundance standard 丰度标准星accreting binary 吸积双星accretion column 吸积柱accretion flow 吸积流accretion mound 吸积堆accretion ring 吸积环accretion stream 吸积流acoustic mode 声模active binary 活动双星active chromosphere binary 活动⾊球双星active chromosphere star 活动⾊球星active optics 主动光学actuator 促动器Adams ring 亚当斯环adaptive optics ⾃适应光学additional perturbation 附加摄动AGB, asymptotic giant branch 渐近巨星⽀Alexis, Array of Low-Energy X-ray 〈阿列克希斯〉低能 X 射线Imaging Sensors 成象飞⾏器AM Herculis star 武仙 AM 型星amplitude spectrum 变幅谱angular elongation 距⾓anonymous galaxy 未名星系anonymous object 未名天体anti-jovian point 对⽊点annular-total eclipse 全环⾷aperture photometry 孔径测光APM, Automated Photographic Measuring 〈APM〉底⽚⾃动测量仪systemapoapse 远质⼼点apoapse distance 远质⼼距apogalacticon 远银⼼点apomartian 远⽕点apparent association 表观成协apparent luminosity function 视光度函数apparent superluminal motion 视超光速运动apsidal advance 拱线进动apsidal precession 拱线进动Arcturus group ⼤⾓星群area image sensor ⾯成象敏感器area photometry ⾯源测光area spectroscopy ⾯源分光argument of pericentre 近⼼点幅⾓ASCA, Advanced Satellite for Cosmology 〈ASCA〉宇宙学和天体物理学and Astrophysics ⾼新卫星asteroidal dynamics ⼩⾏星动⼒学asteroidal resonance ⼩⾏星共振asteroid family ⼩⾏星族asteroid-like object 类⼩⾏星天体asteroseismology 星震学astration 物质改造astroparticle physics 天⽂粒⼦物理学astrostatistics 天⽂统计学asymptotic branch 渐近⽀asymptotic branch giant 渐近⽀巨星atmospheric parameter ⼤⽓参数ATNT, Australia Telescope National 澳⼤利亚国⽴望远镜FacilityATT, Advanced Technology Telescope 〈ATT〉⾼新技术望远镜automated measuring machine 天⽂底⽚⾃动测量仪automatic photooelectric telescope ⾃动光电测光望远镜( APT )AXAF, Advanced X-ray Astrophysical ⾼新X射线天体物理台FacilityBaade's window 巴德窗Baade—Wesselink analysis 巴德—韦塞林克分析Baade—Wesselink mass 巴德—韦塞林克质量Baade—Wesselink method 巴德—韦塞林克⽅法Baade—Wesselink radius 巴德—韦塞林克半径background galaxy 背景星系Barnard's galaxy ( NGC 6822 ) 巴纳德星系barycentric dynamical time ( TDB ) 质⼼⼒学时Belinda 天卫⼗四Bianca 天卫⼋bidimensional spectrography ⼆维摄谱bidimensional spectroscopy ⼆维分光Big-Bang nucleosynthesis ⼤爆炸核合成binarity 成双性binary asteroid 双⼩⾏星binary flare star 耀变双星binary millisecond pulsar 毫秒脉冲双星binary protostar 原双星bioastronomy ⽣物天⽂学bipolar jet 双极喷流bipolar outflow 偶极外向流bipolar planetary nebula 双极⾏星状星云blazar 耀变体blazarlike activity 类耀活动blazarlike object 耀变体Black-eye galaxy ( M 64 ) ⿊眼星系BL Lacertae object 蝎虎天体BL Lacertid 蝎虎天体blue compact galaxy ( BCG ) 蓝致密星系blue straggler 蓝离散星bolometric albedo 热反照率bolometric light curve 全波光变曲线bolometric temperature 热温度Bootes void 牧夫巨洞bow-shock nebula ⼸形激波星云box photometry ⽅格测光broad-band imaging 宽波段成象broad-line radio galaxy ( BLRG ) 宽线射电星系buried channel CCD 埋沟型 CCDButterfly nebula 蝴蝶星云BY Draconis star 天龙 BY 型星BY Draconis variable 天龙 BY 型变星CAMC, Carlsberg Automatic Meridian 卡尔斯伯格⾃动⼦午环Circlecannibalism 吞⾷cannibalized galaxy 被吞星系cannibalizing galaxy 吞⾷星系cannibalizing of galaxies 星系吞⾷carbon dwarf 碳矮星Cassegrain spectrograph 卡焦摄谱仪Cassini 〈卡西尼〉⼟星探测器Cat's Eye nebula ( NGC 6543 ) 猫眼星云CCD astronomy CCD 天⽂学CCD camera CCD 照相机CCD photometry CCD 测光CCD spectrograph CCD 摄谱仪CCD spectrum CCD 光谱celestial clock 天体钟celestial mechanician 天体⼒学家celestial thermal background 天空热背景辐射celestial thermal background radiation 天空热背景辐射central overlap technique 中⼼重迭法Centaurus arm 半⼈马臂Cepheid distance 造⽗距离CFHT, Canada-Franch-Hawaii Telecope 〈CFHT〉望远镜CGRO, Compton Gamma-Ray Observatory 〈康普顿〉γ射线天⽂台chaos 混沌chaotic dynamics 混沌动⼒学chaotic layer 混沌层chaotic region 混沌区chemically peculiar star 化学特殊星Christmas Tree cluster ( NGC 2264 ) 圣诞树星团chromosphere-corona transition zone ⾊球-⽇冕过渡层chromospheric activity ⾊球活动chromospherically active banary ⾊球活动双星chromospherically active star ⾊球活动星chromospheric line ⾊球谱线chromospheric matirial ⾊球物质chromospheric spectrum ⾊球光谱CID, charge injected device CID、电荷注⼊器件circular solution 圆轨解circumnuclear star-formation 核周产星circumscribed halo 外接⽇晕circumstellar dust disk 星周尘盘circumstellar material 星周物质circumsystem material 双星周物质classical Algol system 经典⼤陵双星classical quasar 经典类星体classical R Coronae Borealis star 经典北冕 R 型星classical T Tauri star 经典⾦⽜ T 型星Clementine 〈克莱芒蒂娜〉环⽉测绘飞⾏器closure phase imaging 锁相成象cluster centre 团中⼼cluster galaxy 团星系COBE, Cosmic Background Explorer 宇宙背景探测器coded mask imaging 编码掩模成象coded mask telescope 编码掩模望远镜collapsing cloud 坍缩云cometary burst 彗暴cometary dynamics 彗星动⼒学cometary flare 彗耀cometary H Ⅱ region 彗状电离氢区cometary outburst 彗爆发cometary proplyd 彗状原⾏星盘comet shower 彗星⾬common proper-motion binary 共⾃⾏双星common proper-motion pair 共⾃⾏星对compact binary galaxy 致密双重星系compact cluster 致密星团; 致密星系团compact flare 致密耀斑composite diagram method 复合图法composite spectrum binary 复谱双星computational astrophysics 计算天体物理computational celestial mechanics 计算天体⼒学contact copying 接触复制contraction age 收缩年龄convective envelope 对流包层cooling flow 冷却流co-orbital satellite 共轨卫星coplanar orbits 共⾯轨道Copernicus 〈哥⽩尼〉卫星coprocessor 协处理器Cordelia 天卫六core-dominated quasar ( CDQ ) 核占优类星体coronal abundance 冕区丰度coronal activity 星冕活动、⽇冕活动coronal dividing line 冕区分界线coronal gas 星冕⽓体、⽇冕⽓体coronal green line 星冕绿线、⽇冕绿线coronal helmet 冕盔coronal magnetic energy 冕区磁能coronal red line 星冕红线、⽇冕红线cosmic abundance 宇宙丰度cosmic string 宇宙弦cosmic void 宇宙巨洞COSMOS 〈COSMOS〉底⽚⾃动测量仪C-O white dwarf 碳氧⽩矮星Cowling approximation 柯林近似Cowling mechnism 柯林机制Crescent nebula ( NGC 6888 ) 蛾眉⽉星云Cressida 天卫九critical equipotential lobe 临界等位瓣cross-correlation method 交叉相关法cross-correlation technique 交叉相关法cross disperser prism 横向⾊散棱镜crustal dynamics 星壳动⼒学cryogenic camera 致冷照相机cushion distortion 枕形畸变cut-off error 截断误差Cyclops project 〈独眼神〉计划D abundance 氘丰度Dactyl 艾卫dark halo 暗晕data acquisition 数据采集decline phase 下降阶段deep-field observation 深天区观测density arm 密度臂density profile 密度轮廓dereddening 红化改正Desdemona 天卫⼗destabiliizing effect 去稳效应dew shield 露罩diagonal mirror 对⾓镜diagnostic diagram 诊断图differential reddening 较差红化diffuse density 漫射密度diffuse dwarf 弥漫矮星系diffuse X-ray 弥漫 X 射线diffusion approximation 扩散近似digital optical sky survey 数字光学巡天digital sky survey 数字巡天disappearance 掩始cisconnection event 断尾事件dish 碟形天线disk globular cluster 盘族球状星团dispersion measure 频散量度dissector 析象管distance estimator 估距关系distribution parameter 分布参数disturbed galaxy 受扰星系disturbing galaxy 扰动星系Dobsonian mounting 多布森装置Dobsonian reflector 多布森反射望远镜Dobsonian telescope 多布森望远镜dominant galaxy 主星系double-mode cepheid 双模造⽗变星double-mode pulsator 双模脉动星double-mode RR Lyrae star 双模天琴 RR 型星double-ring galaxy 双环星系DQ Herculis star 武仙 DQ 型星dredge-up 上翻drift scanning 漂移扫描driving system 驱动系统dumbbell radio galaxy 哑铃状射电星系Du Pont Telescope 杜邦望远镜dust ring 尘环dwarf carbon star 碳矮星dwarf spheroidal 矮球状星系dwarf spheroidal galaxy 矮球状星系dwarf spiral 矮旋涡星系dwarf spiral galaxy 矮旋涡星系dynamical age 动⼒学年龄dynamical astronomy 动⼒天⽂dynamical evolution 动⼒学演化Eagle nebula ( M 16 ) 鹰状星云earty cluster 早型星系团early earth 早期地球early planet 早期⾏星early-stage star 演化早期星early stellar evolution 恒星早期演化early sun 早期太阳earth-approaching asteroid 近地⼩⾏星earth-approaching comet 近地彗星earth-approaching object 近地天体earth-crossing asteroid 越地⼩⾏星earth-crossing comet 越地彗星earth-crossing object 越地天体earth orientation parameter 地球定向参数earth rotation parameter 地球⾃转参数eccentric-disk model 偏⼼盘模型effect of relaxation 弛豫效应Egg nebula ( AFGL 2688 ) 蛋状星云electronographic photometry 电⼦照相测光elemental abundance 元素丰度elliptical 椭圆星系elliptical dwarf 椭圆矮星系emulated data 仿真数据emulation 仿真encounter-type orbit 交会型轨道enhanced network 增强络equatorial rotational velocity ⾚道⾃转速度equatorium ⾏星定位仪equipartition of kinetic energy 动能均分eruptive period 爆发周期Eskimo nebula ( NGC 2392 ) 爱斯基摩星云estimated accuracy 估计精度estimation theory 估计理论EUVE, Extreme Ultraviolet Explorer 〈EUVE〉极紫外探测器Exclamation Mark galaxy 惊叹号星系Exosat 〈Exosat〉欧洲 X 射线天⽂卫星extended Kalman filter 扩充卡尔曼滤波器extragalactic jet 河外喷流extragalactic radio astronomy 河外射电天⽂extrasolar planet 太阳系外⾏星extrasolar planetary system 太阳系外⾏星系extraterrestrial intelligence 地外智慧⽣物extreme helium star 极端氦星Fabry-Perot imaging spectrograph 法布⾥-珀罗成象摄谱仪Fabry-Perot interferometry 法布⾥-珀罗⼲涉测量Fabry-Perot spectrograph 法布⾥-珀罗摄谱仪face-on galaxy 正向星系face-on spiral 正向旋涡星系facility seeing ⼈为视宁度fall 见落陨星fast pulsar 快转脉冲星fat zero 胖零Fermi normal coordinate system 费⽶标准坐标系Fermi-Walker transportation 费⽶-沃克移动fibre spectroscopy 光纤分光field centre 场中⼼field galaxy 场星系field pulsar 场脉冲星filter photography 滤光⽚照相观测filter wheel 滤光⽚转盘find 发见陨星finder chart 证认图finderscope 寻星镜first-ascent giant branch 初升巨星⽀first giant branch 初升巨星⽀flare puff 耀斑喷焰flat field 平场flat field correction 平场改正flat fielding 平场处理flat-spectrum radio quasar 平谱射电类星体flux standard 流量标准星flux-tube dynamics 磁流管动⼒学f-mode f 模、基本模following limb 东边缘、后随边缘foreground galaxy 前景星系foreground galaxy cluster 前景星系团formal accuracy 形式精度Foucaultgram 傅科检验图样Foucault knife-edge test 傅科⼑⼝检验fourth cosmic velocity 第四宇宙速度frame transfer 帧转移Fresnel lens 菲涅尔透镜fuzz 展云Galactic aggregate 银河星集Galactic astronomy 银河系天⽂Galactic bar 银河系棒galactic bar 星系棒galactic cannibalism 星系吞⾷galactic content 星系成分galactic merge 星系并合galactic pericentre 近银⼼点Galactocentric distance 银⼼距galaxy cluster 星系团Galle ring 伽勒环Galilean transformation 伽利略变换Galileo 〈伽利略〉⽊星探测器gas-dust complex ⽓尘复合体Genesis rock 创世岩Gemini Telescope ⼤型双⼦望远镜Geoalert, Geophysical Alert Broadcast 地球物理警报⼴播giant granulation 巨⽶粒组织giant granule 巨⽶粒giant radio pulse 巨射电脉冲Ginga 〈星系〉X 射线天⽂卫星Giotto 〈乔托〉空间探测器glassceramic 微晶玻璃glitch activity ⾃转突变活动global change 全球变化global sensitivity 全局灵敏度GMC, giant molecular cloud 巨分⼦云g-mode g 模、重⼒模gold spot ⾦斑病GONG, Global Oscillation Network 太阳全球振荡监测GroupGPS, global positioning system 全球定位系统Granat 〈⽯榴〉号天⽂卫星grand design spiral 宏象旋涡星系gravitational astronomy 引⼒天⽂gravitational lensing 引⼒透镜效应gravitational micro-lensing 微引⼒透镜效应great attractor 巨引源Great Dark Spot ⼤暗斑Great White Spot ⼤⽩斑grism 棱栅GRO, Gamma-Ray Observatory γ射线天⽂台guidscope 导星镜GW Virginis star 室⼥ GW 型星habitable planet 可居住⾏星Hakucho 〈天鹅〉X 射线天⽂卫星Hale Telescope 海尔望远镜halo dwarf 晕族矮星halo globular cluster 晕族球状星团Hanle effect 汉勒效应hard X-ray source 硬 X 射线源Hay spot 哈伊斑HEAO, High-Energy Astronomical 〈HEAO〉⾼能天⽂台Observatoryheavy-element star 重元素星heiligenschein 灵光Helene ⼟卫⼗⼆helicity 螺度heliocentric radial velocity ⽇⼼视向速度heliomagnetosphere ⽇球磁层helioseismology ⽇震学helium abundance 氦丰度helium main-sequence 氦主序helium-strong star 强氦线星helium white dwarf 氦⽩矮星Helix galaxy ( NGC 2685 ) 螺旋星系Herbig Ae star 赫⽐格 Ae 型星Herbig Be star 赫⽐格 Be 型星Herbig-Haro flow 赫⽐格-阿罗流Herbig-Haro shock wave 赫⽐格-阿罗激波hidden magnetic flux 隐磁流high-field pulsar 强磁场脉冲星highly polarized quasar ( HPQ ) ⾼偏振类星体high-mass X-ray binary ⼤质量 X 射线双星high-metallicity cluster ⾼⾦属度星团;⾼⾦属度星系团high-resolution spectrograph ⾼分辨摄谱仪high-resolution spectroscopy ⾼分辨分光high - z ⼤红移Hinotori 〈⽕鸟〉太阳探测器Hipparcos, High Precision Parallax 〈依巴⾕〉卫星Collecting SatelliteHipparcos and Tycho Catalogues 〈依巴⾕〉和〈第⾕〉星表holographic grating 全息光栅Hooker Telescope 胡克望远镜host galaxy 寄主星系hot R Coronae Borealis star ⾼温北冕 R 型星HST, Hubble Space Telescope 哈勃空间望远镜Hubble age 哈勃年龄Hubble distance 哈勃距离Hubble parameter 哈勃参数。