Effect of Al Doping on the Electric and Dielectric Properties of CaCu3Ti4O12

高三英语论文观点论证分析单选题30题(带答案)1.Which of the following is a good way to introduce a paper on environmental protection?A.In recent years, environmental protection has become a hot topic.B.As we all know, environmental protection is very important.C.Environmental protection, which is a crucial issue, should be taken seriously.D.With the development of society, environmental protection is increasingly concerned.答案：A。

解析：选项A 直接指出近年来环境保护成为热门话题，简洁明了地引入了关于环境保护的论文主题。

选项B“众所周知”这种表述比较老套。

选项C 句子结构稍显复杂，不太适合作为开头引入。

选项D“随着社会的发展”表述比较宽泛，没有选项A 直接聚焦于环境保护这个主题。

2.When introducing a paper on history, which one is more appropriate?A.History is full of mysteries and wonders.B.As we all know, history is an important subject.C.History, which has a long and rich tradition, is fascinating.D.In the field of knowledge, history plays a significant role.答案：C。

RANDOM WALK TO GRAPHENENobel Lecture, December 8, 2010byANDRE K. GEIMSchool of Phys i cs and Astronomy, The Un i vers i ty of Manchester, Oxford Road, Manchester M13 9PL, Un i ted K i ngdom.If one wants to understand the beaut i ful phys i cs of graphene, they w i ll be spo i led for cho i ce w i th so many rev i ews and popular sc i ence art i cles now ava i lable. I hope that the reader w i ll excuse me i f on th i s occas i on I recommend my own wr i t i ngs [1–3]. Instead of repeat i ng myself here, I have chosen to descr i be my tw i sty sc i ent ific road that eventually led to the Nobel Pr i ze. Most parts of th i s story are not descr i bed anywhere else, and i ts t i me-l i ne covers the per i od from my PhD i n 1987 to the moment when our 2004 paper, recogn i sed by the Nobel Comm i ttee, was accepted for publ i cat i on. The story naturally gets denser i n events and explanat i ons towards the end. Also, i t prov i des a deta i led rev i ew of pre-2004 l i terature and, w i th the benefit of h i nds i ght, attempts to analyse why graphene has attracted so much i nter-est. I have tr i ed my best to make th i s art i cle not only i nformat i ve but also easy to read, even for non-phys i c i sts.ZOMBIE MANAGEMENTMy PhD thes i s was called “Invest i gat i on of mechan i sms of transport relaxa-t i on i n metals by a hel i con resonance method”. All I can say i s that the stuff was as i nterest i ng at that t i me as i t sounds to the reader today. I publ i shed five journal papers and fin i shed the thes i s i n five years, the offic i al durat i on for a PhD at my i nst i tut i on, the Inst i tute of Sol i d State Phys i cs.Web of Sc i ence so-berly reveals that the papers were c i ted tw i ce, by co-authors only. The subject was dead a decade before I even started my PhD. However, every cloud has i ts s i lver l i n i ng, and what I un i quely learned from that exper i ence was that I should never torture research students by offer i ng them “zomb i e” projects. After my PhD, I worked as a staff sc i ent i st at the Inst i tute of M i cro-electron i cs Technology, Chernogolovka, wh i ch belongs to the Russ i an Academy of Sc i ences. The Sov i et system allowed and even encouraged jun i or staff to choose the i r own l i ne of research. After a year of pok i ng i n d i fferent d i rect i ons, I separated research-w i se from my former PhD superv i sor, V i ctor Petrashov, and started develop i ng my own n i che. It was an exper i mental system that was both new and doable, wh i ch was nearly an oxymoron, tak i ng i nto account the scarce resources ava i lable at the t i me at Sov i et researchi nst i tutes. I fabr i cated a sandw i ch cons i st i ng of a th i n metal film and a super-conductor separated by a th i n i nsulator. The superconductor served only to condense an external magnet i c field i nto an array of vort i ces, and th i s h i ghly i nhomogeneous magnet i c field was projected onto the film under i nvest i ga-t i on. Electron transport i n such a m i croscop i cally i nhomogeneous field (vary i ng on a subm i cron scale) was new research terr i tory, and I publ i shed the first exper i mental report on the subject [4], wh i ch was closely followed by an i ndependent paper from S i mon Bend i ng [5]. It was an i nterest i ng and reasonably i mportant n i che, and I cont i nued study i ng the subject for the next few years, i nclud i ng a spell at the Un i vers i ty of Bath i n 1991 as a postdoctoral researcher work i ng w i th S i mon.Th i s exper i ence taught me an i mportant lesson: that i ntroduc i ng a new exper i mental system i s generally more reward i ng than try i ng to find new phenomena w i th i n crowded areas. The chances of success are much h i gher where the field i s new. Of course, the fantast i c results one or i g i nally hopes for are unl i kely to mater i al i se, but, i n the process of study i ng any new system, someth i ng or i g i nal i nev i tably shows up.ONE MAN’S JUNK, ANOTHER MAN’S GOLDIn 1990, thanks to V i taly Ar i stov, d i rector of my Inst i tute i n Chernogolovka at the t i me, I rece i ved a s i x month v i s i t i ng fellowsh i p from the Br i t i sh Royal Soc i ety. Laurence Eaves and Peter Ma i n from Nott i ngham Un i vers i ty k i ndly agreed to accept me as a v i s i tor. S i x months i s a very short per i od for exper i mental work, and c i rcumstances d i ctated that I could only study de-v i ces read i ly ava i lable i n the host laboratory. Ava i lable were subm i cron GaAs w i res left over from prev i ous exper i ments, all done and dusted a few years earl i er. Under the c i rcumstances, my exper i ence of work i ng i n a poverty-str i cken Sov i et academy was helpful. The samples that my hosts cons i dered pract i cally exhausted looked l i ke a gold ve i n to me, and I started work i ng 100 hours per week to explo i t i t. Th i s short v i s i t led to two Phys. Rev. Letters of decent qual i ty [6,7], and I often use th i s exper i ence to tease my younger colleagues. When th i ngs do not go as planned and people start compla i n i ng, I provoke them by procla i m i ng ‘there i s no such th i ng as bad samples; there are only bad postdocs/students’. Search carefully and you w i ll always find someth i ng new. Of course, i t i s better to avo i d such exper i ences and explore new terr i tor i es, but even i f one i s fortunate enough to find an exper i mental system as new and exc i t i ng as graphene, met i culousness and perseverance allow one to progress much further.The pace of research at Nott i ngham was so relentless and, at the same t i me so i nsp i r i ng, that a return to Russ i a was not an opt i on. Sw i mm i ng through Sov i et treacle seemed no less than wast i ng the rest of my l i fe. So at the age of th i rty-three and w i th an h-i ndex of 1 (latest papers not yet publ i shed), I entered the Western job market for postdocs. Dur i ng the next four years I moved between d i fferent un i vers i t i es, from Nott i ngham to Copenhagen to Bath and back to Nott i ngham. Each move allowed me to get acqua i nted w i thyet another top i c or two, s i gn ificantly broaden i ng my research hor i zons. The phys i cs I stud i ed i n those years could be broadly descr i bed as mesoscop i c and i nvolved such systems and phenomena as two-d i mens i onal electron gases (2DEGs), quantum po i nt contacts, resonant tunnell i ng and the quantum Hall effect (QHE), to name but a few. In add i t i on, I became fam i l i ar w i th GaAlAs heterostructures grown by molecular beam ep i taxy (MBE) and i mproved my expert i se i n m i crofabr i cat i on and electron-beam l i thography, technolog i es I had started learn i ng i n Russ i a. All these elements came together to form the foundat i on for the successful work on graphene a decade later.DUTCH COMFORTBy 1994 I had publ i shed enough qual i ty papers and attended enough con-ferences to hope for a permanent academ i c pos i t i on. When I was offered an assoc i ate professorsh i p at the Un i vers i ty of N i jmegen, I i nstantly se i zed upon the chance of hav i ng some secur i ty i n my new post-Sov i et l i fe. The first task i n N i jmegen was of course to establ i sh myself. To th i s end, there was no start-up and no m i crofabr i cat i on to cont i nue any of my prev i ous l i nes of re-search. As resources, I was offered access to magnets, cryostats and electron i c equ i pment ava i lable at N i jmegen’s H i gh F i eld Magnet Laboratory, led by Jan Kees Maan. He was also my formal boss and i n charge of all the money. Even when I was awarded grants as the pr i nc i pal i nvest i gator (the Dutch fund i ng agency FOM was generous dur i ng my stay i n N i jmegen), I could not spend the money as I w i shed. All funds were d i str i buted through so-called ‘work i ng groups’ led by full professors. In add i t i on, PhD students i n the Netherlands could formally be superv i sed only by full professors. Although th i s probably sounds strange to many, th i s was the Dutch academ i c system of the 1990s. It was tough for me then. For a couple of years, I really struggled to adjust to the system, wh i ch was such a contrast to my joyful and product i ve years at Nott i ngham. In add i t i on, the s i tuat i on was a b i t surreal because outs i de the un i vers i ty walls I rece i ved a warm-hearted welcome from everyone around, i nclud i ng Jan Kees and other academ i cs.St i ll, the research opportun i t i es i n N i jmegen were much better than i n Russ i a and, eventually, I managed to surv i ve sc i ent ifically, thanks to help from abroad. Nott i ngham colleagues (i n part i cular Mohamed Hen i n i) prov i ded me w i th 2DEGs that were sent to Chernogolovka, where Sergey Dubonos, a close colleague and fr i end from the 1980s, m i crofabr i cated requested dev i ces. The research top i c I eventually found and later focused on can be referred to as mesoscop i c superconduct i v i ty. Sergey and I used m i cron-s i zed Hall bars made from a 2DEG as local probes of the magnet i c field around small superconduct i ng samples. Th i s allowed measurements of the i r magnet i sat i on w i th accuracy suffic i ent to detect not only the entry and ex i t of i nd i v i dual vort i ces but also much more subtle changes. Th i s was a new exper i mental n i che, made poss i ble by the development of an or i g i nal techn i que of ball i st i c Hall m i cromagnetometry [8]. Dur i ng the next fewyears, we explo i ted th i s n i che area and publ i shed several papers i n Nature and Phys. Rev. Letters wh i ch reported a paramagnet i c Me i ssner effect, vort i ces carry i ng fract i onal flux, vortex configurat i ons i n confined geometr i es and so on. My w i fe Ir i na Gr i gor i eva, an expert i n vortex phys i cs [9], could not find a job i n the Netherlands and therefore had plenty of t i me to help me w i th conquer i ng the subject and wr i t i ng papers. Also, Sergey not only made the dev i ces but also v i s i ted N i jmegen to help w i th measurements. We establ i shed a very product i ve modus operand i where he collected data and I analysed them w i th i n an hour on my computer next door to dec i de what should be done next.A SPELL OF LEVITYThe first results on mesoscop i c superconduct i v i ty started emerg i ng i n 1996, wh i ch made me feel safer w i th i n the Dutch system and also more i nqu i s i-t i ve. I started look i ng around for new areas to explore. The major fac i l i ty at N i jmegen’s H i gh F i eld Lab was powerful electromagnets. They were a major headache, too. These magnets could prov i de fields up to 20 T, wh i ch was somewhat h i gher than 16 to 18 T ava i lable w i th the superconduct i ng magnets that many of our compet i tors had. On the other hand, the elec-tromagnets were so expens i ve to run that we could use them only for a few hours at n i ght, when electr i c i ty was cheaper. My work on mesoscop i c super-conduct i v i ty requ i red only t i ny fields (< 0.01T), and I d i d not use the electro-magnets. Th i s made me feel gu i lty as well as respons i ble for com i ng up w i th exper i ments that would just i fy the fac i l i ty’s ex i stence. The only compet i t i ve edge I could see i n the electromagnets was the i r room temperature (T) bore. Th i s was often cons i dered as an extra d i sadvantage because research i n condensed matter phys i cs typ i cally requ i res low, l i qu i d-hel i um T. The con-trad i ct i on prompted me, as well as other researchers work i ng i n the lab, to ponder on h i gh-field phenomena at room T. Unfortunately, there were few to choose from.Eventually, I stumbled across the mystery of so-called magnet i c water. It i s cla i med that putt i ng a small magnet around a hot water p i pe prevents format i on of scale i ns i de the p i pe. Or i nstall such a magnet on a water tap, and your kettle w i ll never suffer from chalky depos i ts. These magnets are ava i lable i n a great var i ety i n many shops and on the i nternet. There are also hundreds of art i cles wr i tten on th i s phenomenon, but the phys i cs beh i nd i t rema i ns unclear, and many researchers are scept i cal about the very ex i stence of the effect [10]. Over the last fifteen years I have made several attempts to i nvest i gate “magnet i c water” but they were i nconclus i ve, and I st i ll have noth i ng to add to the argument. However, the ava i lab i l i ty of ultra-h i gh fields i n a room T env i ronment i nv i ted lateral th i nk i ng about water. Bas i cally, i f magnet i c water ex i sted, I thought, then the effect should be clearer i n 20 T rather than i n typ i cal fields of <0.1 T created by standard magnets.W i th th i s i dea i n m i nd and, allegedly, on a Fr i day n i ght, I poured water i ns i de the lab’s electromagnet when i t was at i ts max i mum power. Pour i ngwater i n one's equ i pment i s certa i nly not a standard sc i ent ific approach, and I cannot recall why I behaved so ‘unprofess i onally’. Apparently, no one had tr i ed such a s i lly th i ng before, although s i m i lar fac i l i t i es ex i sted i n several places around the world for decades. To my surpr i se, water d i d not end up on the floor but got stuck i n the vert i cal bore of the magnet. Humberto Carmona, a v i s i t i ng student from Nott i ngham, and I played for an hour w i th the water by break i ng the blockage w i th a wooden st i ck and chang i ng the field strength. As a result, we saw balls of lev i tat i ng water (F i gure 1). Th i s was awesome. It took l i ttle t i me to real i se that the phys i cs beh i nd was good old d i amagnet i sm. It took much longer to adjust my i ntu i t i on to the fact that the feeble magnet i c response of water (~10–5), b i ll i ons of t i mes weaker than that of i ron, was suffic i ent to compensate the earth’s grav i ty. Many colleagues, i nclud i ng those who worked w i th h i gh magnet i c fields all the i r l i ves, were flabbergasted, and some of them even argued that th i s was a hoax.I spent the next few months demonstrat i ng magnet i c lev i tat i on to colleagues and v i s i tors, as well as try i ng to make a ‘non-boffin’i llustrat i on for th i s beaut i ful phenomenon. Out of the many objects that we had float i ng i ns i de the magnet, i t was the i mage of a lev i tat i ng frog (F i gure 1) that started the med i a hype. More i mportantly, though, beh i nd all the med i a no i se, th i s i mage found i ts way i nto many textbooks. However qu i rky, i t has become a beaut i ful symbol of ever-present d i amagnet i sm, wh i ch i s no longer perce i ved to be extremely feeble. Somet i mes I am stopped at conferences by people excla i m i ng “I know you! Sorry, i t i s not about graphene. I start my lectures w i th show i ng your frog. Students always want to learn how i t could fly.” The frog story, w i th some i ntr i cate phys i cs beh i nd the stab i l i ty of d i amagnet i c lev i tat i on, i s descr i bed i n my rev i ew i n Phys i cs Today [11].F i gure 1. Lev i tat i ng moments i n N i jmegen. Left – Ball of water (about 5 cm i n d i ameter) freely floats i ns i de the vert i cal bore of an electromagnet. R i ght – The frog that learned to fly. Th i s i mage cont i nues to serve as a symbol show i ng that magnet i sm of ‘nonmagnet i c th i ngs’, i nclud i ng humans, i s not so negl i g i ble. Th i s exper i ment earned M i chael Berry and me the 2000 Ig Nobel Pr i ze. We were asked first whether we dared to accept th i s pr i ze, and I take pr i de i n our sense of humour and self-deprecat i on that we d i d.FRIDAY NIGHT EXPERIMENTSThe lev i tat i on exper i ence was both i nterest i ng and add i ct i ve. It taught me the i mportant lesson that pok i ng i n d i rect i ons far away from my i mmed i ate area of expert i se could lead to i nterest i ng results, even i f the i n i t i al i deas were extremely bas i c. Th i s i n turn i nfluenced my research style, as I started mak i ng s i m i lar exploratory detours that somehow acqu i red the name ‘Fr i day n i ght exper i ments’. The term i s of course i naccurate. No ser i ous work can be accompl i shed i n just one n i ght. It usually requ i res many months of lateral th i nk i ng and d i gg i ng through i rrelevant l i terature w i thout any clear i dea i n s i ght. Eventually, you get a feel i ng – rather than an i dea – about what could be i nterest i ng to explore. Next, you g i ve i t a try, and normally you fa i l. Then, you may or may not try aga i n. In any case, at some moment you must dec i de (and th i s i s the most d i fficult part) whether to cont i nue further efforts or cut losses and start th i nk i ng of another exper i ment. All th i s happens aga i nst the backdrop of your ma i n research and occup i es only a small part of your t i me and bra i n.Already i n N i jmegen, I started us i ng lateral i deas as under- and post-graduate projects, and students were always exc i ted to buy a p i g i n a poke. Kostya Novoselov, who came to N i jmegen as a PhD student i n 1999, took part i n many of these projects. They never lasted for more than a few months, i n order not to jeopard i se a thes i s or career progress i on. Although the enthus i asm i nev i tably van i shed towards the end, when the pred i ctable fa i lures mater i al i sed, some students later confided that those exploratory detours were i nvaluable exper i ences.Most surpr i s i ngly, fa i lures somet i mes fa i led to mater i al i se. Gecko tape i s one such example. Acc i dentally or not, I read a paper descr i b i ng the mechan i sm beh i nd the amaz i ng cl i mb i ng ab i l i ty of geckos [12]. The phys i cs i s rather stra i ghtforward. Gecko’s toes are covered w i th t i ny ha i rs. Each ha i r attaches to the oppos i te surface w i th a m i nute van der Waals force (i n the nN range), but b i ll i ons of ha i rs work together to create a form i dable attract i on suffic i ent to keep geckos attached to any surface, even a glass ce i l i ng. In part i cular, my attent i on was attracted by the spat i al scale of the i r ha i rs. They were subm i cron i n d i ameter, the standard s i ze i n research on mesoscop i c phys i cs. After toy i ng w i th the i dea for a year or so, Sergey Dubonos and I came up w i th procedures to make a mater i al that m i m i cked a gecko’s ha i ry feet. He fabr i cated a square cm of th i s tape, and i t exh i b i ted notable adhes i on [13]. Unfortunately, the mater i al d i d not work as well as a gecko’s feet, deter i orat i ng completely after a couple of attachments. St i ll, i t was an i mportant proof-of-concept exper i ment that i nsp i red further work i n the field. Hopefully, one day someone w i ll develop a way to repl i cate the h i erarch i cal structure of gecko’s setae and i ts self-clean i ng mechan i sm. Then gecko tape can go on sale.BETTER TO BE WRONG THAN BORINGWh i le prepar i ng for my lecture i n Stockholm, I comp i led a l i st of my Fr i day n i ght exper i ments. Only then d i d I real i se a stunn i ng fact. There were two dozen or so exper i ments over a per i od of approx i mately fifteen years and, as expected, most of them fa i led m i serably. But there were three h i ts: lev i tat i on, gecko tape and graphene. Th i s i mpl i es an extraord i nary success rate: more than 10%. Moreover, there were probably near-m i sses, too. For example, I once read a paper [14] about g i ant d i amagnet i sm i n FeGeSeAs alloys, wh i ch was i nterpreted as a s i gn of h i gh-T superconduct i v i ty. I asked Lamarches for samples and got them. Kostya and I employed ball i st i c Hall magnetometry to check for g i ant d i amagnet i sm but found noth i ng, even at 1 K. Th i s happened i n 2003, well before the d i scovery of i ron pn i ct i de superconduct i v-i ty, and I st i ll wonder whether there were any small i nclus i ons of a supercon-duct i ng mater i al wh i ch we m i ssed w i th our approach. Another m i ss was an attempt to detect “heartbeats” of i nd i v i dual l i v i ng cells. The i dea was to use 2DEG Hall crosses as ultrasens i t i ve electrometers to detect electr i cal s i gnals due to phys i olog i cal act i v i ty of i nd i v i dual cells. Even though no heartbeats were detected wh i le a cell was al i ve, our sensor recorded huge voltage sp i kes at i ts “last gasp” when the cell was treated w i th excess alcohol [15]. Now I attr i bute th i s near-m i ss to the unw i se use of yeast, a very dormant m i cro-organ i sm. Four years later, s i m i lar exper i ments were done us i ng embryon i c heart cells and – what a surpr i se – graphene sensors, and they were successful i n detect i ng such b i oelectr i cal act i v i ty [16].Frankly, I do not bel i eve that the above success rate can be expla i ned by my lateral i deas be i ng part i cularly good. More l i kely, th i s tells us that pok i ng i n new d i rect i ons, even randomly, i s more reward i ng than i s generally perce i ved. We are probably d i gg i ng too deep w i th i n establ i shed areas, leav i ng plenty of unexplored stuff under the surface, just one poke away. When one dares to try, rewards are not guaranteed, but at least i t i s an adventure.THE MANCUNIAN WAYBy 2000, w i th mesoscop i c superconduct i v i ty, d i amagnet i c lev i tat i on and four Nature papers under my belt, I was well placed to apply for a full professorsh i p. Colleagues were rather surpr i sed when I chose the Un i vers i ty of Manchester, decl i n i ng a number of seem i ngly more prest i g i ous offers. The reason was s i mple.M i ke Moore, cha i rman of the search comm i ttee, knew my w i fe Ir i na when she was a very successful postdoc i n Br i stol rather than my co-author and a part-t i me teach i ng lab techn i c i an i n N i jmegen. He suggested that Ir i na could apply for the lecturesh i p that was there to support the professorsh i p. After s i x years i n the Netherlands, the i dea that a husband and w i fe could offic i ally work together had not even crossed my m i nd. Th i s was the dec i s i ve factor. We apprec i ated not only the poss i b i l i ty of sort i ng out our dual career problems but also felttouched that our future colleagues cared. We have never regretted the move.So i n early 2001, I took charge of several d i lap i dated rooms stor i ng anc i ent equ i pment of no value, and a start-up grant of £100K. There were no central fac i l i t i es that I could explo i t, except for a hel i um l i quefier. No problem. I followed the same rout i ne as i n N i jmegen, comb i n i ng help from other places, espec i ally Sergey Dubonos. The lab started shap i ng up surpr i s i ngly qu i ckly. W i th i n half a year, I rece i ved my first grant of £500K, wh i ch allowed us to acqu i re essent i al equ i pment. Desp i te be i ng consumed w i th our one year old daughter, Ir i na also got her start i ng grant a few months later. We i nv i ted Kostya to jo i n us as a research fellow (he cont i nued to be offic i ally reg i stered i n N i jmegen as a PhD student unt i l 2004 when he defended h i s thes i s there). And our group started generat i ng results that led to more grants that i n turn led to more results.By 2003 we publ i shed several good-qual i ty papers i nclud i ng Nature, Nature Mater i als and Phys. Rev. Letters, and we cont i nued beefing up the labora-tory w i th new equ i pment. Moreover, thanks to a grant of £1.4M (research i nfrastructure fund i ng scheme masterm i nded by the then sc i ence m i n i ster Dav i d Sa i nsbury), Ern i e H i ll from the Department of Computer Sc i ences and I managed to set up the Manchester Centre for Mesosc i ence and Nanotechnology. Instead of pour i ng the w i ndfall money i nto br i cks-and-mortar, we ut i l i sed the ex i st i ng clean room areas (~250 m2) i n Computer Sc i ences. Those rooms conta i ned obsolete equ i pment, and i t was thrown away and replaced w i th state-of-the-art m i crofabr i cat i on fac i l i t i es, i nclud i ng a new electron-beam l i thography system. The fact that Ern i e and I are most proud of i s that many groups around the world have more expens i ve fac i l i t i es but our Centre has cont i nuously, s i nce 2003, been produc i ng new structures and dev i ces. We do not have a posh horse here that i s for show, but rather a draft horse that has been work i ng really hard.Whenever I descr i be th i s exper i ence to my colleagues abroad, they find i t d i fficult to bel i eve that i t i s poss i ble to establ i sh a fully funct i onal labora-tory and a m i crofabr i cat i on fac i l i ty i n less than three years and w i thout an astronom i cal start-up grant. If not for my own exper i ence, I would not bel i eve i t e i ther. Th i ngs progressed unbel i evably qu i ckly. The Un i vers i ty was support i ve, but my greatest thanks are reserved spec ifically for the respons i ve mode of the UK Eng i neer i ng and Phys i cal Sc i ences Research Counc i l (EPSRC). The fund i ng system i s democrat i c and non-xenophob i c. Your pos i t i on i n an academ i c h i erarchy or an old-boys network counts for l i ttle. Also, ‘v i s i onary i deas’ and grand prom i ses to ‘address soc i al and econom i c needs’ play l i ttle role when i t comes to the peer rev i ew. In truth, the respons i ve mode d i str i butes i ts money on the bas i s of a recent track record, whatever that means i n d i fferent subjects, and the fund i ng normally goes to researchers who work both effic i ently and hard. Of course, no system i s perfect, and one can always hope for a better one. However, paraphras i ng W i nston Church i ll, the UK has the worst research fund i ng system, except for all the others that I am aware of.THREE LITTLE CLOUDSAs our laboratory and Nanotech Centre were shap i ng up, I got some spare t i me for th i nk i ng of new research detours. Gecko tape and the fa i led attempts w i th yeast and quas i-pn i ct i des took place dur i ng that t i me. Also, Serge Morozov, a sen i or fellow from Chernogolovka, who later became a regular v i s i-tor and i nvaluable collaborator, wasted h i s first two v i s i ts on study i ng magnet i c water. In the autumn of 2002, our first Manchester PhD student, Da J i ang, arr i ved, and I needed to i nvent a PhD project for h i m. It was clear that for the first few months he needed to spend h i s t i me learn i ng Engl i sh and gett i ng acqua i nted w i th the lab. Accord i ngly, as a starter, I suggested to h i m a new lateral exper i ment. It was to make films of graph i te ‘as th i n as poss i ble’ and, i f successful, I prom i sed we would then study the i r ‘mesoscop i c’ propert i es. Recently, try i ng to analyse how th i s i dea emerged, I recalled three badly shaped thought clouds.One cloud was a concept of ‘metall i c electron i cs’. If an external electr i c field i s appl i ed to a metal, the number of charge carr i ers near i ts surface changes, so that one may expect that i ts surface propert i es change, too. Th i s i s how modern sem i conductor electron i cs works. Why not use a metal i nstead of s i l i con? As an undergraduate student, I wanted to use electr i c field effect (EFE) and X-ray analys i s to i nduce and detect changes i n the latt i ce constant. It was naïve because s i mple est i mates show that the effect would be negl i g i ble. Indeed, no d i electr i c allows fields much h i gher than 1V/nm, wh i ch translates i nto max i mum changes i n charge carr i er concentrat i on n at the metal surface of about 1014 per cm2. In compar i son, a typ i cal metal (e.g., Au) conta i ns ~1023 electrons per cm3 and, even for a 1 nm th i ck film, th i s y i elds relat i ve changes i n n and conduct i v i ty of ~1%, leav i ng as i de much smaller changes i n the latt i ce constant.Prev i ously, many researchers asp i red to detect the field effect i n metals. The first ment i on i s as far back as 1902, shortly after the d i scovery of the electron. J. J. Thomson (1906 Nobel Pr i ze i n Phys i cs) suggested to Charles Mott, the father of Nev i ll Mott (1977 Nobel Pr i ze i n Phys i cs), to look for the EFE i n a th i n metal film, but noth i ng was found [17]. The first attempt to measure the EFE i n a metal was recorded i n sc i ent ific l i terature i n 1906 [18]. Instead of a normal metal, one could also th i nk of sem i metals such as b i smuth, graph i te or ant i mony wh i ch have a lot fewer carr i ers. Over the last century, many researchers used B i films (n ~1018 cm–3) but observed only small changes i n the i r conduct i v i ty [19,20]. Aware of th i s research area and w i th exper i ence i n GaAlAs heterostructures, I was cont i nuously, albe i t casually, look i ng for other cand i dates, espec i ally ultra-th i n films of superconductors i n wh i ch the field effect can be ampl ified i n prox i m i ty to the superconduct i ng trans i t i on [21,22]. In N i jmegen, my enthus i asm was once sparked by learn i ng about nm-th i ck Al films grown by MBE on top of GaAlAs heterostructures but, after est i mat i ng poss i ble effects, I dec i ded that the chances of success were so poor i t was not worth try i ng.Carbon nanotubes were the second cloud hang i ng around i n the late。

高二英语科技成果单选题50题(带答案)1.The development of technology has brought many____changes to our lives.A.significantB.insignificantC.occasionalD.rare答案：A。

本题考查形容词辨析。

“significant”表示重大的、显著的；“insignificant”表示不重要的；“occasional”表示偶尔的；“rare”表示罕见的。

科技的发展给我们的生活带来了很多重大的变化，所以选A。

2.Modern technology has created____tools for communication.A.efficientB.inefficientC.limitedeless答案：A。

“efficient”表示高效的；“inefficient”表示低效的；“limited”表示有限的；“useless”表示无用的。

现代科技创造了高效的通讯工具，所以选A。

3.The new scientific discovery is of____importance.A.greatB.littleC.someD.no答案：A。

本题考查固定搭配。

“of great importance”表示非常重要。

新的科学发现非常重要，所以选A。

4.Technological advances have led to____improvements in healthcare.A.dramaticB.slightC.noD.rare答案：A。

“dramatic”表示巨大的、戏剧性的；“slight”表示轻微的；“no”表示没有；“rare”表示罕见的。

科技进步给医疗保健带来了巨大的改善，所以选A。

5.The latest technology product is____in design.A.advancedB.backwardC.old-fashionedD.outdated答案：A。

The Effect of Diet on AttentionThe connection between nutrition and concentration has been known for some time Harvard Medical School states, “What you eat directly affects the structure and function of your brain, and ultimately your mood.”To explore this, retailer Approved Food commissioned the surveys, which asked students, “What do you find most difficult about lectures?” and “What would you normally eat before a lecture?”The results suggest that students having a problem concentrating on their studies may wish to look at their dietary intake in a bid to improve focus.According to the survey, students who did eat prior to their studies confessed （坦白）to choosing food that doesn't have the optimum nutritional value.Poor diet--a factor synonymous with student living-is often a result of restricted finances. However, online grocery retailer Approved Food provides a cost-effective solution for students in the name of short-dated foods.Dan Cluderay, from Approved Food, says, “Short-dated food that has reached or is nearing its ‘best before’ date is still absolutely fine to eat; however, many retailers discard this food as waste. This is obviously a ridiculous waste of perfectly good food. We specialise in sourcing high-quality short-dated foods and supplying them at a reduced price to our customers, enabling anyone-not least those on tighter budgets such as students-to access a wide variety of top brand nutritious foods at a fraction of the cost.”Other difficulties encountered by students received far fewer votes than concentration. “Understanding the subject matter” came second with 13.3%, significantly smaller than the 72. 4% who cited concentration.This confirms that concentration is a serious issue for students-but one that can easily be addressed with improved nutrition.Increased awareness of cheaper shopping options would undoubtedly improve the quality of student diets, ultimately having a positive knock-on effect on their grades.There is a general misconception regarding the freshness guidelines on UKpackaging, as Dan concludes, “Just because a food product is nearing or past its ‘best before’ date does not mean that it is not fit for consumption, the date simply indicates when the flavours are likely to reach their peak. Goods carrying a use by date--usually fresh meats and dairy products--are the ones that could be harmful if consumed past the date therefore, we do not stock anything carrying a ‘use by’ date.”Reading ComprehensionChoose the correct answer for each question.1. What is Approved Food?A. A food producer.B. A retailer.C. A food brand.D. A non-profit organization.2. Which is often a result of restricted finances with students according to the article?A. Poor diet.B. Bad health.C. Malnutrition.D. Poor concentration.3. What do many retailers deal with short-dated food that has reached its “best before” date?A. Sell this food at low prices.B. Get rid of this food as waste.C. Hand out this food to beggars.D. Give this food to the welfare home.4. How can students improve their concentration?A. By eating fresh food only.B. By eating top brand nutritious food.C. By improving nutrition.D. By improving the quality of lectures.5. Which of the following is NOT mentioned in this article?A. Approved Food purchases high-quality short-dated foods.B. Students' concentration is highly connected with nutrition.C. Approved Food does not stock food carrying a “use by” date.D. Eating more high-protein food helps to improve concentration.Vocabulary基础词nutrition n. 营养（作用）；滋养；营养的补给记词根记忆：nutri（滋养）+tion→营养explore v. 探索；探测，勘探；探究记词根记忆：ex（出）+plor（大喊）+e→把隐藏的大喊岀来→探索例Tech companies should explore ways of improving creativity. 科技公司应该探索提高创新的方法。

First-principles calculations of electronic structure and optical properties of Boron-doped ZnO with intrinsicdefectsYen-Chun Peng,Chieh-Cheng Chen,Hsuan-Chung Wu ⇑,Jong-Hong LuDepartment of Materials Engineering,Ming Chi University of Technology,New Taipei 24301,Taiwana r t i c l e i n f o Article history:Received 11August 2014Received in revised form 27October 2014Accepted 27October 2014Available online 15November 2014Keywords:First principles B-doped ZnO Intrinsic defectElectronic structure Optical propertya b s t r a c tThis study adopted first-principles calculations to evaluate the effects of intrinsic defects on the elec-tronic structure and optical properties of Boron-doped ZnO (BZO).Four types of defect were considered:non-defective (B Zn ),Zn vacancies (V Zn ),O vacancies (V O ),and interstitial Zn (Zn i ).Calculations of forma-tion energy illustrate that O-rich conditions tend to induce V Zn ,while O-poor conditions tend to induce V O and Zn i .With respect to electric properties,V Zn defects in BZO decrease carrier concentration as well as mobility,which consequently decreases the conductivity of BZO.The existence of V O or Zn i defects in BZO leads to n-type conductive characteristics and increases the optical band gap.The existence of Zn i defects in BZO also increases the effective mass,which decreases the mobility and conductivity of BZO.As for the optical properties,the introduction of V Zn to BZO leads to an increase in transmittance in the visible light region,but a decrease in the UV region.The introduction of intrinsic V O and Zn i defects to BZO leads to a significant decrease in transmittance in the visible as well as UV regions.The calculated results were also compared with experimental data from the literature.Ó2014Elsevier B.V.All rights reserved.1.IntroductionZnO is an abundant,non-toxic material with a wide band gap (3.37eV)and transparent properties under visible light.ZnO has recently attracted considerable attention as an alternative for Tin-doped In 2O 3(ITO),which is currently the most common choice of transparent conductive oxide for a variety of applications [1,2].The resistivity of pure ZnO is on the order of 10À2X -cm,which is far higher than that of ITO (10À4X -cm order).A great deal of research has gone into enhancing the conductivity of ZnO through the addition of various dopants,which can mainly be divided into metals [3–5]and non-metals [6,7].B-doped ZnO (BZO)thin film shows considerable promise for its superior photoelectric proper-ties and stability [8,9].Many groups have investigated the effects of process parameters on the electric and optical properties of BZO thin film,with the aim of optimizing performance [7–13].Miyata et al.[7]indicated that the transmittance of BZO thin film could be improved through the introduction of O 2gas from 0sccm to 10sccm.David et al.[10]reported that annealing temperature and atmosphere strongly affect the conductivity of BZO.Yang et al.[11]concluded that the low oxygen partial pressure during deposition increases the carrier density of oxygen vacancies,which leads to a strong decline in resistivity.However,resistivity in sam-ples produced under the high oxygen partial pressure is far higher than in samples deposited under low oxygen partial pressure,which suggests the existence of p-type carriers of Zinc vacancies in films grown under high oxygen partial pressure.Patil et al.[12]synthesized B-doped ZnO powders using a mechanochemical method.The photoluminescence (PL)spectra at room temperature is an indication that a greater number of oxygen vacancies exist in nonmetal-doped ZnO,compared to pure ZnO.In the fabrication of BZO microrods,Yılmaz et al.[13]investigated the influence of B diffusion doping on optical emission and defect formation.PL spec-tra results revealed that the intensity of the deep level visible band emission increases with an increase in annealing time,which implies a significant increase in the concentration of intrinsic defects.As outlined above,various process conditions influence the type and number of intrinsic defects with a subsequent influence on the electric and optical properties of BZO.Gaining a comprehensive understanding of the electric and optical characteristics of BZO would require in-depth study into the effects of intrinsic defects on the properties of BZO.First-principles calculations can provide information concerning materials at the microscopic scale to eluci-date the connection between structure and properties.It is well known that the use of conventional density functional theory/10.1016/j.optmat.2014.10.0580925-3467/Ó2014Elsevier B.V.All rights reserved.⇑Corresponding author at:Department of Materials Engineering,Ming Chi University of Technology,84Gungjuan Road,Taishan,New Taipei 24301,Taiwan.Tel.:+8862290898994675;fax:+886229084091.E-mail address:hcwu@.tw (H.-C.Wu).(DFT)leads to a considerable underestimation of the calculated band gap in ZnO [14–16].In our previous study [17],we used the DFT plus Hubbard U (DFT +U)method to avoid underestimat-ing the band gap.This approach reduced the differences in calcu-lated band gap and lattice constant to within 1%of the experimental values.The current study extended the utilization of the DFT +U method to calculate and analyze the effects of intrin-sic defects (V Zn ,V O ,and Zn i )on the formation energy,crystal struc-ture,electronic structure,and optical properties of BZO.These results clarify the connections among the fabrication process,structure,and properties of BZO,for use in determining the criteria for future material designs.2.Calculation methodsThis study considered a 2Â2Â2supercell of a Wurtzite ZnO,including 16Zn atoms and 16O atoms,as shown in Fig.1.A B-monodoping model was constructed by substituting one Zn atom (number 1site)with one B atom (B Zn model),which correspond to the B concentrations of 6.25at.%.We also considered three intrinsic defects in the B Zn structure,in which Zn vacancies(B Zn V Zn ),O vacancies (B Zn V O ),and interstitial Zn (B Zn Zn i )are repre-sented as 2,3,and 4,respectively.The V Zn ,V O ,and Zn i concentra-tions corresponds to doping levels of 6.25, 6.25,and 5.88at.%,respectively.The defect concentration could be reduced using a larger supercell for the real systems;however,this study was lim-ited with regard to computer resources.Therefore,the properties of the defects calculated from a 2Â2Â2ZnO supercell such as this could be used as qualitative analysis.1432ZnO BTable 1Formation energy and optimized structure of BZO with varying intrinsic defects.Formation energy (eV)Optimized structure O-richO-poor Zn–O (Å)B–O (Å)4V (%)ZnO –– 1.981––B Zn3.750.39 1.996 1.526À3.1B Zn V Zn 5.68 5.81 1.993 1.530À3.3B Zn V O 7.550.70 1.995 1.521À5.3B Zn Zn i10.513.662.0031.5174.74.5 eV2.15 eV3.25 eV4.68 eV4.41 eV(a)(b)(c)(d)Band structures of B-doped ZnO for (a)B Zn ,(b)B Zn V Zn ,(c)B Zn V O models.Y.-C.Peng et al./Optical Materials 39(2015)34–3935All models presented in this study were developed using CASTEP software [18].Structural optimization was performed on each model before calculating properties.The Monkhorst–Pack scheme [19]K-points grid sampling in the supercells was set at 4Â4Â2.Electron–ion interactions were modeled using the ultrasoft pseudo-potential method [20].The valence configurations of the atoms were 4s 23d 10for Zn,2s 22p 4for O,and 2s 22p 1for B.The elec-tron wave functions were expanded in plane wave with an energy cutoff of 380eV.In the structural optimization process,the change in energy,maximum force,maximum stress,and maximum displacement tolerances were set at 10À5eV/atom,0.03eV/Å,0.05GPa,and 0.001Å,respectively.The energy convergence crite-rion for the self-consistent field was set at 10À6eV.To describe the electronic structures more accurately,we adopted the DFT +U d +U p method [21],in which the U d value for Zn-3d and the U p value for O-2p orbitals were set at 10and 7eV,respectively.The band structures,band gaps,and Zn-3d orbital locations of pure ZnO,which were used for the selection of U d and U p values,can be referenced in our previous research [17,22].3.Results and discussion 3.1.Optimized structureThe average bond lengths and volume difference ratio,as obtained from geometric optimization,are summarized in Table 1.In pure ZnO,each Zn atom is bonded to its three horizontal and one vertical oxygen neighbors.The average bond length of Zn-O is 1.981Åand optimized lattice constants are a =b =3.249Åandc =5.232Å,which are in agreement with the experimental values of a =b =3.249,c =5.206Å[23].Following the substitution of one B atom for one Zn atom (B Zn model),the Zn–O bond length is longer than that of B–O (1.526Å).This is because the B 3+radius (0.27Å)is smaller than that of Zn 2+(0.74Å)[24].Therefore,the cell volume of B Zn model shrinks,which is consistent with the experi-mental results [25].Clearly,the presence of Zn or O vacancies in BZO also leads to a shrinkage in volume.Conversely,the presence of interstitial Zn leads to a longer Zn–O length and expansion in volume.3.2.Formation energyTo examine the relative stability of BZO with intrinsic defects in neutral charge state,the defect formation energy can be expressed as follows:[26,27]E f ðD Þ¼E tot ðD ÞÀE tot ðZnO ÞþXn i l ið1Þwhere E tot (ZnO)and E tot (D )are the total energy in pure ZnO and in the defective systems,respectively.n i is the number of i atoms removed from or added to the supercell.If an atom is removed from the supercell,n i is positive,otherwise is negative.l i is the chemical potential of atom i .Formation energy depends on the growth envi-ronment during the preparation process,which can be O-rich or O-poor (Zn-rich).In thermo-dynamic equilibrium,D l Zn +D l O =D H f (ZnO),where D H f (ZnO)represents the formation enthalpy of ZnO.For the chemical potential of B,this study adopted the relation of 2D l B +3D l O 6D H f (B 2O 3)under O-rich conditions and l B =l B(bulk)under O-poor conditions,where D H f (B 2O 3)represents the(a)(b)(c)(d)V ZnV OZn iBZnOO density difference for (a)B Zn ,(b)B Zn V Zn ,(c)B Zn V O ,and (d)B Zn Zn i models.The red,orange,yellow,green,and interpretation of the references to color in this figure legend,the reader is referred to the web version 36Y.-C.Peng et al./Optical Materials 39(2015)34–39formation enthalpy of B2O3.D l i represents the chemical potential of atom i referred to as the elemental solid/gas of l i(bulk/molecule).It is well known that a defective structure with lower formation energy forms more readily and denotes an increased occurrence of defects.Table1presents a summary of the calculated formation energy of BZO with various intrinsic defects,based on the neutral charge state.With the existence of B Zn,E f(B Zn V Zn)<E f(B Zn V O)<E f (B Zn Zn i)under O-rich conditions,implying that O-rich conditions are more likely to induce the formation of V Zn,followed by V O and Zn i.Under O-poor conditions,E f(B Zn V O)<E f(B Zn Zn i)<E f (B Zn V Zn),which implies that O-poor conditions are more likely to induce the formation of V O.As a result,process conditions,such as O2gasflow rate and substrate temperature,largely determine the type of intrinsic defects that form in BZO during preparation. The occurrence of V O is far more likely under a low-O atmosphere, and V Zn is more likely to occur under a high-O atmosphere.For the sake of comparison,we also calculated the formation energy of a single intrinsic defect(V Zn and V O)in pure ZnO.The calculated values of E f(V Zn)and E f(V O)are3.09and4.33eV under O-rich conditions and6.58and0.84eV under O-poor conditions. Thus,we can see that the formation energy of a Zn vacancy from pure ZnO(E f(V Zn))is greater than that obtained from BZO(E f(B Zn V Zn)ÀE f(B Zn)=1.93eV under O-rich conditions and5.42eV under O-poor conditions).This demonstrates that Zn vacancies form more easily in BZO than in ZnO.These results are similar to those calculated for O vacancies,which implies that B-doping facilitates the formation of V Zn and V O.Previous studies [12]obtained similar results,indicating that a greater number of oxygen vacancies or defects exist in BZO than in pure ZnO.3.3.Electronic structureTo clarify the influence of intrinsic defects on the electronic structure of BZO,we calculated the band structure,difference in charge density,and density of states(DOS),as shown in Figs.2–4,respectively.In our previous study[17,22],the calculated band structure revealed a band gap of3.37eV in pure ZnO,which is in excellent agreement with values obtained in experiments.In the present study,we focus on the properties of BZO with intrinsic defects.Fig.2presents the band structures for BZO with various intrin-sic defect models.The Fermi level indicated by the dotted line was set to zero.Fig.2(a)shows the situation in which a Zn atom in pure ZnO is replaced by a B atom,in which the Fermi level shifts from the valence band(VB)maximum to the bottom of the conduction band(CB),resulting in a shallow donor level at the bottom of the CB.The shallow donor level at B doping causes an increase in the optical band gap to4.5eV at B concentration of6.25at.%,which is well known as the Burstein-Moss effect[28].The definitionof Fig.4.Density of states for(a)B Zn,(b)B Zn V Zn,(c)B Zn V O,and(d)B Zn Zn i models.optical band gap is from the top offor n-type semiconducting materials and the bottom of conduction band for materials.Similar tendencies were experiment-based studies[29,30].Asin the vicinity of B impurities appears atoms.The calculated Mulliken bond and B–O bonds are0.371and0.658, that a B–O bond is more covalent than Mulliken bond population represents characteristics).As shown in Fig.4(a),to the bottom of CB are the Zn-4s anda few O-2s and O-2p orbitals.The main extra electron tofill up the CBM.According to the results calculated regarded as an intrinsic defect under the B Zn V Zn model(Fig.2(b)),when donor levels coexist,the empty states produced trons from the B Zn donor level,resulting level as well as the formation of p-type band gap of B Zn V Zn can be narrowed to eration of conduction electrons requires energy from the Fermi level to the CB, be required in the B Zn model.Thus,in may lead to a decrease in the carrier known that mobility is related to the time.The relaxation time could not be software and was assumed as a defects in BZO.The followingeffects of the effective mass on thenear the Fermi level appear nearlyof carriers with a smaller curvature The larger effective mass is related toTherefore,V Zn defects in BZO reduce both carrier concentration as well as mobility,which consequently increases the resistivity of BZO.Fig.3(b)shows that the O atoms surrounding a Zn vacancy gain fewer electrons(green color),implying the occurrence of a number of empty states of O atoms.These empty states are O-2p orbitals near the Fermi level,as shown in Fig.4(b).V O and Zn i can be regarded as intrinsic defects in an O-poor environment.Fig.2(c)and(d)show the band structures in B Zn V O and B Zn Zn i models,in which n-type conductive characteristics appear and the optical band gap increases to4.68eV and4.41eV, respectively.One shallow donor state and one deep donor state occur in these two models.In the B Zn V O model,the deep donor level is probably the charge remaining in the oxygen vacancy (Fig.3(c));in the B Zn Zn i model,it is probably the covalence charge in the vicinity of the interstitial Zn atom(Zn i)(Fig.3(d)).Fig.4(c) and(d)show that the shallow donor level in both models origi-nated from B doping,whereas the deep donor level in the B Zn V O and B Zn Zn i models originated from the addition of V O and Zn i, respectively.The deep donor level in the B Zn V O model comprises mainly Zn and O atoms;however,in the B Zn Zn i model,it also includes B atom(B-2s and B-2p states).The shallow donor states provide conduction electrons;however,the deep donor states may contribute less to the increase in carrier concentration.Qual-itatively,the curvature of the energy band near the Fermi level in the B Zn Zn i model is smaller than that in the B Zn V O model.There-fore,Zn i defects present in BZO increase the effective mass,which may consequently decrease the mobility and conductivity of BZO.3.4.Optical propertiesThe optical properties can be described via the dielectric func-tion e(x)=e1(x)+i e2(x)[31].The imaginary part of the dielectric function e2(x)is calculated as follows:e2¼2e2pX e0Xk;v;cu ckuÁrj j u v k2d E ckÀE vkÀxÀÁð2Þwhere e is the electronic charge;X is the unit cell volume;u is the vector defining the polarization of the incident electricfield;x is the frequency of light;and u c k and u v k are the wave functions of the conduction and valence bands,respectively.Fig.5(a)shows the e2(x)of BZO with various intrinsic defects. In the B Zn model,a blue-shift in the intrinsic absorption edge occurred due to an enlarged optical band gap as compared with ZnO.The shallow donor levels mentioned in Section3.3resulted in an absorption peak at1.2eV.The absorption peaks in the intrin-sic defect models were as follows:B Zn V Zn(0.3eV),B Zn V O(1.5eV), and B Zn Zn i(0.9eV).These peaks resulted in enhanced absorption in the visible range.The peak of B Zn V Zn is the lowest,which can probably be attributed to the transition between occupied states and unoccupied states near the Fermi level.In the B Zn V O,and B Zn Zn i models,the absorption in the visible range may be the result of a shift from the shallow and deep donor occupied states to the unoccupied states of the conduction band.Fig.5(b)presents the transmittance of BZO under various defec-tive models.Table2presents the calculated values for average transmittance associated with each model under UV and visible light.It is should be noted that the calculated results were based on the doping levels of B(6.25at.%),V Zn(6.25at.%)V O(6.25at.%), and Zn i(5.88at.%).The average transmittance of pure ZnO is89.2 %in the visible region and65.6%in the UV region.Fig.5(b)shows that the incorporation of B into ZnO decreased transmittance in the range of800–1200nm(infrared region)and400–800nm(visible light region),but increased transmittance in the range of200–400nm(UV region),compared with pure ZnO.When V Zn was introduced to BZO,transmittance in the visible light region was Optical properties of BZO with varying intrinsic defects.(a)Imaginary dielectric function,(b)Transmittance.38increased,but transmittance in the UV region decreased,compared with BZO.When V O or Zn i was introduced to BZO,the transmit-tance of visible as well as UV light declined significantly,which implies that transmittance could be reduced by employing a low-O environment for fabrication.This may explain why trans-mittance is enhanced by an increase in oxygen pressure during processing[7].4.ConclusionsThis study used the DFT+U method to investigate the influence of intrinsic defects on the formation energy,crystal structure,elec-tronic structure,and optical properties of BZO.Our results revealed that the formation energy of V Zn is lowest under O-rich conditions and the formation energy of V O is lowest under O-poor conditions. 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姓名：王德军性别：男出生年月：1951-8-11 工作单位：吉林大学所在院系：化学学院职称：教授行政职务：招生专业：070304物理化学（含：化学物理）研究领域：功能材料光化学与光物理是否兼职：否指导博士生总数：指导硕士生总数：目前博士生数：目前硕士生数：个人简介主要学历：1973.09--1977.01 吉林大学化学系物理化学本科1980.09--1983.07 吉林大学化学系物理化学硕士1985.09--1989.07 吉林大学化学系物理化学博士主要学术经历：1987.10-1988.10 日本京都大学工学部学习1991.01 吉林大学化学系副教授1991.01-1992.12 吉林大学化学站博士后研究1996.10 至今吉林大学化学学院教授2001.05 至今吉林大学化学学院博士生导师总的研究方向是“功能材料光化学与光物理”，具体为：1、利用表面光电压谱技术研究功能材料的光生电荷的转移过程与光电活性的关系2、建立Kelvin探针测量系统，相对准确的固体功能材料的功函；对部分纳米材料（TiO2、ZnO、CdS、CdSe等）的功函和它们表面电势进行研究；利用有机染料敏化半导体纳米材料，在光诱导下研究表面和界面电荷转移过程。

利用Kelvin技术研究纳米材料的功函和研究外来粒子对半导体纳米材料的表面电势的影响。

3、利用瞬态表面光伏技术，对一些功能材料的光生电荷性质进行了研究。

在系统制备纳米功能材料和光电功能体系活性评价的基础上，重点是利用瞬态光伏方法研究光生电荷转移过程与光电活性的关系。

4、利用表面光电流方法来研究染料敏化ZnO纳米粒子的光生电荷界面转移过程，并研究O2、H2O、甲醛等吸附质对表面光电流的影响。

在能量匹配的情况下，染料受光激发产生的光生电子可以注入到纳米ZnO导带，使其在可见区产生表面光电流。

使用宽度为10 ?m 左右的ITO 梳状电极，在激光光源或由分光系统产生的单色光激发下，对有机染料敏化的纳米ZnO进行表面光电流的测试，通过表面光电流的研究了解纳米ZnO的表面电荷状态和界面电荷转移的信息。

The properties and behavior ofelectric fieldsElectric fields are a fundamental concept in physics, and they play an essential role in our daily lives. They are invisible but pervasive, and they govern the way in which electrically charged particles interact with each other. In this article, we will explore the properties and behavior of electric fields, including their origin, intensity, and effects on charged particles.Electric fields are created by the presence of charges. When a positive charge is placed in space, it creates an electric field that radiates outwards in all directions. This electric field exerts a force on any charges that happen to be nearby, either attracting them towards the positive charge or repelling them away from it, depending on their sign.The intensity of an electric field is characterized by its strength, which is measured in volts per meter (V/m). The strength of an electric field is determined by the distribution of charges that create it. If there are many charges close together, the field will be strong, whereas if the charges are far apart, the field will be weak. Moreover, the strength of an electric field diminishes as one moves farther away from the source charge, following an inverse-square law.One important property of electric fields is that they are vector quantities. This means that they have both magnitude and direction, and they can be represented by vectors. The direction of an electric field is defined as the direction in which a positive test charge would move if it were placed in the field. Consequently, the direction of the field at any point is always radially outward for a positive charge and radially inward for a negative charge.Another important property of electric fields is that they can be superimposed. This means that if there are two or more charges creating electric fields in the same region of space, the resulting electric field is the vector sum of the individual fields. For example, ifthere are two positively charged objects positioned near each other, the resulting electric field will be the sum of the electric fields created by each object individually.One of the most interesting aspects of electric fields is their effects on charged particles. Charged particles experience a force whenever they are placed in such a field. This force is given by the equation F=qE, where F is the force, q is the charge of the particle, and E is the electric field strength. This equation tells us that the force on a charged particle is directly proportional to both the charge of the particle and the strength of the electric field.One important consequence of this equation is that particles with the same charge will repel each other when placed in an electric field. For example, if two positively charged particles are placed in an electric field, they will experience a repulsive force that will push them away from each other. In contrast, negatively charged particles will attract each other when placed in an electric field.Moreover, electric fields can generate currents of charged particles. When a conductor (such as a copper wire) is placed in an electric field, the electrons in the conductor experience a force that causes them to move. This movement of electrons constitutes an electric current, which can be used to power electronic devices.In summary, electric fields are a fundamental concept in physics that play a crucial role in our daily lives. They are created by charges and characterized by strength, direction, and vector nature. They can be superimposed and have a significant impact on charged particles, including the forces they experience and the currents they generate. Understanding the properties and behavior of electric fields is essential for both theoretical and practical applications, including electronics, power generation, and communication.。

Applied Surface Science 277 (2013) 105–110Contents lists available at SciVerse ScienceDirectApplied SurfaceSciencej o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p s u scElectrodeposited Ag nanoparticles on TiO 2nanorods for enhanced UV visible light photoreduction CO 2to CH 4Dan Kong,Jeannie Ziang Yie Tan,Fei Yang,Jieliang Zeng,Xiwen Zhang ∗State Key Laboratory of Silicon Materials,Department of Materials Science and Engineering,ZheJiang University,Hangzhou 310027,PR Chinaa r t i c l ei n f oArticle history:Received 26February 2013Received in revised form 31March 2013Accepted 3April 2013Available online 17 April 2013Keywords:Ag nanoparticle Electrodeposition CO 2photoreduction TiO 2nanorodsa b s t r a c tWe employed the double-potentiostatic methodology to electrodeposit Ag nanoparticles on oriented single-crystalline rutile TiO 2nanorods synthesized by hydrothermal method.The synthesized composites were used as the photocatalyst to reduce CO 2to CH 4under UV irradiation,and tested by SEM,XRD,TEM,XPS,UV–vis and photoluminescence.Deposition with Ag nanoparticles was observed to enhance the pho-tocatalytic activity (≈1.5–2.64␮mol (g catal h)−1)up to 5times with respect to undecorated TiO 2nanorods (≈0.5␮mol (g catal h)−1).The increase in the CH 4yield was correlated with the surface morphology and structure of TiO 2nanorods.© 2013 Elsevier B.V. All rights reserved.1.IntroductionAs is known to everyone,carbon dioxide (CO 2)is one of the major greenhouse gases in recent years because the emissions have sharply increased from fossil fuel combustion over the past few centuries [1].CO 2photoreduction is one promising strategy to switch the rising emissions to renewable energy products such as CO,methane (CH 4),methanol (CH 3OH),formaldehyde (HCOOH)[2].Many researchers have reported that titanium dioxide (TiO 2),TiO 2nanocomposites [3]and other metal oxide catalysts can con-vert CO 2[1,4]to methane or methanol in the presence of water.TiO 2is almost a perfect photocatalyst due to its excellent chemical sta-bility,easy operation,low cost and high catalytic activity.However,the relatively high energy bandgap (3.2eV)limits TiO 2only operat-ing under UV irradiation with a wavelength shorter than 387.5nm [4].An effective resolution to improve the photocatalytic activity is modifying TiO 2by doping with metal impurities,such as noble and transition metals [5–8].For instance,Ag has successfully decorated TiO 2for increasing yields of several photocatalytic reactions [9–14].The advantages of Ag doped or deposited on TiO 2are (i)to modify the surface morphologies and structures of TiO 2,(ii)to improve the electron–hole separation by performing as electron traps [15],(iii)to increase the surface electron activity by localized surface plasma Resonance [16,17].∗Corresponding author.E-mail address:zhangxw@ (X.Zhang).Koˇc i et al.[12]prepared Ag-enriched TiO 2powders by sol–gel process controlled in the reverse micellar environ-ment.The catalysts were tested in CO 2photocatalytic reduction under UV irradiation,where the highest yields of methane was 0.38␮mol g catal −1after 24h of UV irradiation.The Ag-doped sol–gel TiO 2powder possessed higher photocatalytic activity than pure TiO 2.Xingtian Yin et al.[17]probed that Ag–TiO 2nanocomposites were prepared at low temperature in polyethylene glycol solution.TiO 2powders with different Ag contents were employed to decom-pose methyl orange under UV.The rapid separation of photoexcited charge carriers and the surface Plasmon effect of Ag nanoparticles in the visible region contributed to a better photocatalytic activity of samples.However,the sol–gel method to preparing Ag–TiO 2nanocom-posites is not so successful,as the particle size and the spatial distribution is not homogenous compared to the electrochem-ical deposition [18–21].Until now,several methods have been employed such as double potential pulses [20],potential step [22],or cyclic voltammetry scan [23].Depositing Ag nanoparticles onto substrates by double-potentiostatic method is rarely recorded.The double-potentiostatic method can realize a quick nucleation and slow growth under the high nucleation potential and low precur-sor concentration,maintaining a suitable growth rate and obtaining uniform and dense metallic nanoparticles [24,25].In our report,Ag nanoparticles were deposited on TiO 2nanorods by double-potentiostatic methods under different nucleation potentials.Hydrothermal method was used to grow TiO 2nanorods on transparent conductive fluorine-doped tin oxide (FTO)sub-strates.The prepared catalysts were evaluated by photoreduction0169-4332/$–see front matter © 2013 Elsevier B.V. All rights reserved./10.1016/j.apsusc.2013.04.010106 D.Kong et al./Applied Surface Science277 (2013) 105–110CO2under UV irradiation.The results showed that TiO2decorated with Ag nanoparticles improved photocatalytic activity,and Ag deposited TiO2under−1.0V nucleation potential had better perfor-mance for the better size variation and uniform spatial distribution compared to other nucleation potentials.2.Experimental2.1.Preparation of photocatalystsTiO2nanorods were prepared by the hydrothermal method on FTO substrates following the report[26].In the beginning,FTO sub-strates(F:SnO2,Tec15,10 m,Hartford Glass Company)were ultrasonically cleaned for60min in a mixture of deionized water, acetone and2-propanol with volume ratios of1:1:1.Tecdeionized water and concentrated hydrochloric acid(36.5–38%by weight) were mixed(ratio1:1)to reach a total volume of480mL,stirred at ambient conditions for15min,and then stirred for15min after the addition of8mL of titanium butoxide(97%Aldrich).The pre-cursor solution immersed FTO substrates,placed at angle against the wall of the Teflon-liner with the conduction side facing down. TiO2nanorods were growing on FTO at150◦C for20h in an elec-tric oven.After that,the FTO substrates were removed,rinsed completely with deionized water overnight and dried in ambi-ent conditions.0.1M KNO3,0.2mM sodium citrate(C6H5Na3O7) and0.05mM AgNO3made up of aqueous electrolyte.The double-potentiostatic method was used on an electrochemical workstation (ES550,Gaoss Union Technology Co.,Ltd.,Wuhan,China).Based on linear sweep voltammogram of TiO2nanorodsfilm from0.2V to −1.5V at−0.05V/s,the nucleation potential was chosen to deposit Ag atoms ranging from−1.4V to−0.8V for100s and the growth potential was−0.2V for2400s at room temperature(28±1◦C)in a standard three-electrode system.FTO coated with TiO2nanorods was used as working electrode,Pt plate as counter electrode and saturated calomel electrode(SCE)as reference electrode.2.2.Characterization of photocatalystsScanning electron microscopy(SEM,Hitachi S4800)was employed to characterize the surface morphologies of TiO2 nanorods and silver nanoparticles.The crystal structure of the as-prepared samples was examined by X-ray diffraction(XRD)in a X’Pert PRO diffractometer using Cu K␣radiation( =1.5406˚A) from20◦to80◦at a scanning speed of2◦/min.The elemental composition of as-prepared samples was analyzed by X-ray photo-electron spectroscopy(XPS)in a VG ESCALAB Mark II instrument using Mg-K␣excitation source.The amount of spectra,recorded at normal emission with pass energies of0.8eV at300W,was collected from the area under the curve of Ag nanoparticles on TiO2film according to the Handbook of X-ray Photoelectron Spec-troscopy(Physical Electronics Division,Eden Prairie,Minnesota, USA,1979).Microstructure was characterized by transmission electron microscopy(TEM)image on a Philips CM200TEM with an acceleration voltage of160kV.The absorption spectra were recorded with a TU-1901UV–vis spectrophotometer by using bare FTO coated glass as the reference.The photoluminescence spectra were carried out on a FLS920fluorescence spectrometer(Edinburg Instruments Ltd.)using a325nm UV xenon lamp as the excitation source.2.3.Photocatalytic reduction experimentsPhotoreduction of CO2was conducted in a quartz reactor with the as-prepared TiO2film placed at the center of the container bottom.The ultraviolet light irradiation system consisted four 8W UVA lamps with a wavelength of365nm(average intensity:3.25mW cm−2,measured by UVX radiometer,UVP)and located in two groups on opposite sides of the container.The details of the photoreduction process and analytical methods were described in our previous report[27].Ultra-pure gaseous CO2(Air Products, 99.995%)wasflowed through deionized water into the reactor for 30min beforeflowing into the reactor for30min to degas the air from the reactor were automatically analyzed by chromatography (GC/FID,Thermo-Fisher,Trace GC)once an hour during the whole 8h reaction time.Methane was the main organic product from the reactor and the reactor and carbon monoxide was occasionally measured within the detection limits of our method(∼200ppb). Therefore,the direct measure of activity toward CO2photoreduc-tion is referenced to the methane yield.The results were compared with our previous work[27].3.Results and discussionFig.1A(a)shows the linear sweep voltammogram of the elec-trolyte from0.2V to−1.5V at the scanning speed of−0.05V/s. The scanning peak at about−0.6V referred to the electrochemi-cal reduction of Ag nanoparticles[28].The related potential–time and current density–time curves were shown in Fig.1A(b)and (c)respectively.Double-potential methods were implemented to study influence of the nucleation potential for Ag nanoparticles. Such short nucleation periods(100s)and long growth periods (2400s)ensured quick nucleation and slow particle growth pro-cess[29].As the nucleation potential was more negative than −0.8V,the current densities were higher in Fig.1A(c).When the nucleation period prolonged with higher nucleation poten-tial,the Ag precursor concentration was reduced at higher rate. Then the low precursor concentration restrained the growth of the nuclei[21,30].Thus,nucleation potential and deposition time was the key to control the size and diameter of Ag nanoparti-cles if the precursor solution were determined.In other words, the nucleation and growth process of Ag nanoparticles could be realized the controllability of size and diameter by the double-potential.This phenomenon could be further confirmed with SEM.Fig.1B shows the SEM image of the as-prepared TiO2nanorods after deposition with Ag nanoparticles.The top view showed that the nanorods were tetragonal and the lengths being approximately 2␮m.The cross-sectional views(b–e)exhibited a uniform distri-bution of the metal on the surface of the nanorods and the size and diameter of nanoparticles diminishing with increasingly nega-tive nucleation potential.The number of nanoparticles increased as more positions were invoked by increasing the nucleation poten-tial.When the nucleation potential reached−0.8V,agglomeration (van der Waals interactions)or aggregation(chemical bands)of Ag nanoparticles at the top part of the nanorods was observed as shown in Fig.2b.In addition,the high nucleation potential had also destroyed the morphology of TiO2nanorods in Fig.2b.High-resolution XPS spectra of Ag(3d)were displayed in Fig.1C. The peaks observed at363.3and369.4eV referred to Ag3d3/2and 3d5/2electronic states of metallic silver respectively.The6.0eV difference between the binding energy of the peaks was also the characteristic of metallic Ag3d states[6,31].There was no peak for oxidized silver corresponding to Ag2O or AgO observed in the full XPS spectra of all the samples.Thus,XPS data together with SEM images,suggested that Ag nanoparticles were deposited on the nanorods.When the nucleation potential increased from−1.2V to−0.8V,XPS intensity was also increasing,indicating that differ-ent content of Ag nanoparticles were formed on the surfaces.There was no XPS spectra of the Ag component deposited under nuclea-tion potential of−1.4V,which was because that Ag nanoparticles were too small to be observed by XPS.D.Kong et al./Applied Surface Science277 (2013) 105–110107Fig.1.(A)(a)Linear sweep voltammogram of TiO2nanorods from0.2V to−1.5V at−0.05V/s,(b)potential–time curves(−0.8V,−1.0V,−1.2V,−1.4V for100s respectively and−0.2V for2400s)and(c)the corresponding current density–time curves of the double-potentiostatic electrodeposition process,(B)SEM images of top view(a)and cross-sectional view(b)of Ag nanoparticles deposited on TiO2nanorods at nucleation potentials of−0.8V,−1.0V(c),−1.2V(d)and−1.4V(e);(C)XPS spectra of TiO2 nanorods after Ag nanoparticles deposition under the different nucleation potentials.Fig.2.(A)XRD patterns of the TiO2nanorods before(a)and after(b–d)Ag electrodeposition under different nucleation potentials as shown in the legend and(B)TEM pattern and(C)HRTEM pattern of TiO2nanorods after Ag electrodeposition under the nucleation potention of−1.0V.108 D.Kong et al./Applied Surface Science 277 (2013) 105–110Fig.3.(A)UV–vis spectra of TiO 2nanorods before and after Ag deposition at differ-ent nucleation potentials,(B)Fluorescence spectra of pure and Ag–TiO 2nanorods under different nucleation potentials.In Fig.2A XRD patterns showed TiO 2nanorods were rutiles with growth axis in the (101)and (002)directions.The nanorods before and after electrodeposition showed the similar XRD curves.The absence of Ag component on TiO 2nanorods showed that Ag did not enter the TiO 2lattice [32].Ag-deposited TiO 2nanorods at nucleation potential of −1.0V were further investigated by TEM as shown in Fig.2B and C.It clearly displayed that Ag nanoparticles amorphously covered TiO 2nanorods with a diameter of 200nm in Fig.2B.Fig.2C shows the high resolution TEM image of the Ag-TiO 2nanocomposites.The spacing between two adjacent lattice fringes were 0.35nm and 0.24nm,corresponding to the (101)plane of TiO 2and the (111)plane of Ag,respectively.The distinguished interface further confirmed the XRD results.The formation of chem-ical bond between TiO 2and Ag nanoparticles was verified by the continuity of lattice fringes between them.Fig.3A shows the UV–vis spectroscopy of the Ag-deposited TiO 2nanorods at the different nucleation potentials.All the samples had higher absorbance intensities than the unmodified TiO 2nanorod films in the range from 300to 400nm.It was observed a red shift and a broadening peak width with increase of nucleation poten-tial between −1.4V and −1.0V.According to the SEM images,the nanorods-deposited Ag nanoparticles at the nucleation potential of −1.0V exhibited a shorter distance between particles and a larger coverage area of particles without agglomeration,suggesting a better-proportioned metal dispersion than other samples.With the condition of unaggregation,the increase of the deposition amount of Ag on TiO 2nanorods with the increasing nucleation potentials had increased the localized surface Plasmon resonance intensity of Ag nanoparticles [6,33,34,16].Thus,the increase of nucleation potentials leaded to the slight red-shift of absorption edge,which contributed to enhancing photoactivity under visible light [35].Fig.4.(A)Sums of methane yield of prepared catalysts at Ag electrodeposition nucleation potentials of −1.0V and −1.2V under UV irradiation (365nm);(B)pho-tocatalytic process:(a)absorbing electron activity and (b)localized surface plasma resonance of Ag nanoparticles effecting carriers transfer process under irradiation.Fig.3B shows the photoluminescence spectra of pure and Ag-deposited TiO 2nanorods.The fluorescence peaks of Ag–TiO 2were the same as pure TiO 2nanorods in the region of 400–450nm.But the sample after Ag electrodeposition at the nucleation poten-tial of −0.8V had another peak in the region of 350–400nm.This was because that aggregating Ag nanoparticles improved the recombination of excited electron–hole pairs [36].The lower inten-sity of Ag–TiO 2nanorods revealed the decrease in recombining electron–hole pairs on metal-loaded TiO 2nanorod surfaces.The positively charged plasmas of Ag nanoparticles attracted electrons in the conduction band of TiO 2and then increased the capability of electron–hole separation [37].However,if the Ag nanoparticles were adjacent to each other,the separation pairs would have been recombining together very soon even under lower energy [38,39].As the nucleation potential of Ag deposition decreased,the cor-responding intensity of the fluorescence decreased.This suggested that the size and the directional distribution of Ag nanoparticles sig-nificantly influenced the rate of e −/h +separation in semiconductor,as well as re-dox reactions.The photocatalytic activity of Ag–TiO 2with the deposited nucleation potentials of −1.2V and −1.0V was compared to that of pure TiO 2nannorods.Fig.4A exhibited the photocat-alytic results in terms of the methane yield versus reaction time.Both Ag-deposited nanorods exhibited much higher methane total outputs (≈1.5–2.64␮mol (g catal h)−1)than the pure TiO 2(≈0.5␮mol (g catal h)−1),suggesting the metal-decorated TiO 2nanorods could highly enhance photocatalytic activity.The uni-form distribution of Ag nanoparticles on the nanorods improved the separation of photogenerated electrons,as well as providing more electron traps than pure TiO 2nanorods.In addition,the AgD.Kong et al./Applied Surface Science277 (2013) 105–110109nanoparticles as intermediates were more convenient to carry out photocatalytic reductions at superior rates.Both of them were responsible for the excellent photocatalytic performance of Ag–TiO2nanorods.The photocatalytic process of Ag–TiO2nanocomposites,as shown in Fig.4B,was put forward to discuss the above phenomena in detail.In Fig.4B(a)the electron transferred from the excited TiO2nanorods to Ag nanoparticles under UV light,due to the lower Fermi level of Ag(E f=0.4V)than that of TiO2.Thus,more electrons assembled in the Fermi level of the metal and the whole Fermi level of Ag–TiO2was nearer to the conduction band of the TiO2, which brought better reductive power of nanocomposites[40]. The improved electron–hole separation and the enhanced reduc-tive power contributed to the photocatalytic activity of Ag–TiO2 nanocomposites under UV irradiation.In addition,due to the local-ized surface plasma resonance of Ag nanoparticles,incident light induced an electricfield in metal nanoparticles,and those nega-tive charge plasmas and positive charge plasmas were separated, as shown in Fig.4B(b).Then the negative charge plasmas near the valence band reacted with holes and positive charge plasmas near the conduction band reacted with electrons[27].The electrons transferring through the Ag nanoparticles reacted with absorbed oxidants to produce oxygen radicals(O2•−),which reduced recom-bination probability in TiO2[37].Together,Ag-deposited TiO2 nanorods exhibit much better photocatalytic performance than the pure TiO2nanorods in our report.However,as the nucleation potential increased from−1.0V to−0.8V,the Ag nanopartilces aggregated together,which infringed the photocatalytic process. The possible explanation was that the aggregation improved the probability of holes captured by Ag nanoparticles and recombina-tion of electron–hole pairs,thus the poorer photoreduction activity [40].4.ConclusionThis work demonstrated Ag nanoparticles successfully deposited on TiO2nanorods by electrochemical method,could highly improve the photocatalytic activity.Under the different nucleation potentials the different size and distribution of Ag nanoparticles grew up on TiO2nanorods.Photoreduction activity measurements were evaluated by photoreduction of CO2under UV irradiation.The results showed that Ag–TiO2nanocomposites exhibited higher photocatalytic activity of conversion CO2to CH4 with rates up to 2.64␮mol(g catal h)−1than that of pure TiO2, possibly due to their specific structures and morphology assisting in the separation of photogenerated electrons and holes.This report might provide new insights into the design and fabrication of advanced photocatalytic materials with complex hierarchical architectures and enhanced photocatalytic activity. 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a r X i v :c o n d -m a t /0105470v 1 [c o n d -m a t .s u p r -c o n ] 24 M a y 2001Effect of the electric field on a superconducting powder Keshav N.Shrivastava*School of Physics,University of Hyderabad,Hyderabad 500046,India.Abstract.We find that in the presence of an electric field there is an attractive intergrain interaction in superconductors which is small.When charge on the ball is permitted to vary with the ball radius,very large balls can be formed.The pairing energy makes the ball compact and hence reduces the size of the ball compared with the classical value.The ball radius depends on the gap of the superconductor due to Josephson tunneling.Keywords:Electric fields;Josephson interaction*Tel.+91-40-3010811;fax:+91-40-3010145E-mail address:knssp@uohyd.ernet.in (K.N.Shrivastava)1.Introduction.Some time ago,it was found that application of an electricfield produced a second-order interaction which led to anisotropic resistivity with respect to a change in the sign of the electricfield[1].Hence the change in sign of the applied voltage which can be achieved by reversing the battery,resulted into two different values of the resistivity for two different polarities[2].The geometry of the experimental configuration for the application of the electricfield plays an important role.Therefore,another measurement was carried out by Frey et al[3].The second-order term in the potential due to the dipole moment induced by the electricfield leads to a resistivity which is linear in electricfield.At small values of the electricfield,E≤0.3E BD,this prediction[4]is in accord with the experimental measurement.Here E BD is the break-downfield.The break-down voltage is V BD=E BD d with d as the width of thefilm along the c direction.At larger values of the applied voltage, 0.16<E/E BD<0.89,the resistivity is found[4]to depend on the square root of the applied electricfield,R∝E1/2.It is found[4,5]that the density of states depend on the dimensionality so that one-dimensional conduction normal to the surface of thefilm produces the resistivity proportional to the square root of the electricfield.The same result is obtained by more elementary considerations of the Thomas-Fermi screening length.In the present paper,we show that there is an intergrain interaction due to the electric dipole moment which is attractive so that the grains tend to aggregate to make a ball.The Josephson tunneling plays an important role to determine the superconducting surface ten-sion.Wefind that pairing interaction helps in making a compact ball from superconducting grains.The binding of Cooper pairs here,is the same as in the B.C.S.theory except that large grains bind into a big ball.2.TheoryWe assume that the electron coordinates are r i and the charge is−e so that the dipole moment of one ion is p=−e.r i.Since different electrons are located at different coordinates in an ion,we replace the dipole moment by the average value,<p>.The potential energyof a grain due to dipole moment of all of the ions within a grain is given by,V(1) g =Ni=1V(1)=−N i=1E<p i>(1)where the sum is over all ions within a grain and N is the number of ions within the grain. In the case of two grains,i V(1)=−N(1) i=1E<p i1>−N(2) j=1E<p j2>(2) is the potential energy where N(1)is the number of electrons within thefirst grain and N(2) is the number of electrons within the second grain.The states of the grains are given by |02,01>,|n2,n1>,etc.the second-order energy of the system of two grains is,V(2)= i j <n1,n2|Ep i1|01,n2><01,n2|Ep j2|02,01>setting dU/da=0so that,σ=q2/(16πǫo a3)+3E2aǫo/(16π).(5)The charge,q,of the ball is proportional to Eǫo a2.Therefore we assume that q=γEǫo a2 which substituted in the above gives a relation between applied electricfield E and the radius of the ball as,E2a=16πσ/[ǫo(3+γ2)](6)so that the radius of the ball depends on the inverse square of the electricfield,a∝1/E2. This relation seems to be marginally satisfactory for NdBa2Cu3O7−δbut in the case of Bi2Sr2CaCu2O8+δ,the relation is obeyed only for large values of E.For E<0.94kV/mm there is a deviation between the measured values and those calculated from E2a=constant. The measured[7]values of the radius of the ball are smaller than those calculated.None of the three terms in(4)are explicitly dependent on quantum effects.The surface energy is the product of the surface area4πa2and the surface tension but does not have explicit dependence on quantum nature of superconductivity.The Coulomb energy is just the square of the charge devided by the distance and the electrostatic energy also does not involve any quantum effects.If the electrostatic energy was dominant,there will be crystal growth according to the crystallographic symmetry.Since the growth is spherical,according to the crystal symmetries,there will be a texture which minimizes the energy.Case II.We consider that in the free energy(4)the charge depends on the radius of the ball,a.Therefore,we treat the charge as dependent on the radius.The charge q=γEǫo a2 is eliminated from(4)so that the free energy becomes,U=4πa2σ+γ2E2ǫo a32 ǫo E2a3.(7)Minimizing this free energy with respect to the radius of the ball we set dU/da=0which gives,E2a=16πσnot be formed.Case III.Pairing with constant charge.We introduce the pairing energy which is impor-tant for superconductivity and explains the experimentally measured values of the radius of the superconducting ball.The electron-phonon interaction is given by k,k′Dc†k,σc k′,σa k−k′+ h.c.where h.c.stands for the hermitian conjugate of the previous term.The c†k,σ(c k,σ)are the creation(annihilation)operators for electrons of wave vector k and spinσand a†q(a q) for phonons of wave vector q and frequencyω.Here D is the interaction constant.This interaction leads to the attractive potential,V eff=2D2¯hωq c†k↑c†k↓c k↓c k↑Vwhere V is the attractive potential V=−2D2/(¯hωq).The kinetic energy terms of the hamiltonian are slightly renormalized in going from the normal to the superconducting state.However ignoring this small renormalization effect,we can write the average value of the B.C.S.hamiltonian as−2∆2/V=∆2¯hω/D2where∆is the gap energy.Therefore eq.(4)is subject to a quantum correction due to pairing energy.The volume of the ball is4πa3/3and that of a Cooper pair is4πξ3/3so that the number of Cooper pairs is(a/ξ)3.Since∆is the gap in the single-particle dispersion relation the pairing energy inside the superconducting ball is(∆2¯hω/D2)(a/ξ)3.Therefore the energy of the ball becomes,U=4πσa2+q2/(2ǫo a)−a3[∆2¯hω/(D2ξ3)+(1/2)E2ǫo].(10) Minimizing U with respect to the radius of the ball,we set dU/da=0which we solve for the surface tension tofind,σ=q28πD2ξ3+3aE2ǫofor a constant charge on the ball.Substituting the charge of the ball q=γEǫo a2in eq.(11) and solving for the radius we obtain,E2a=16πσ/[ǫo{3+γ2+a1}](12)wherea1=6∆2¯hω/[ǫo D2E2ξ3]=a2(∆2/ξ3).(13) When E is reduced,a1increases and hence the denominator in(12)increases and a reduces. We assume that a1is a small number,a1/(3+γ2)≪1.The eq.(12)then can be written by using the binomial theorem expansion and retaining only thefirst two terms as,E2a=[16πσ/{ǫo(3+γ2)}]{1−a1/(3+γ2)}.(14)The radius of the superconducting ball given by this expression is smaller than that given by (6)due to pairing of electrons.The classical ball is thus compressed by the pairing energy. Since the mass of the ball is independent of the pairing of electrons and the volume is reduced wefind that the density of the ball increases due to pair formation.In the case of strong pairing,a1≫3+γ2,the eq.(12)gives,a=8πσD2ξ3/(3∆2¯hω)(15)and the radius of the superconducting ball becomes independent of the applied electricfield. This means that when charge on the ball is small and the tunneling current is small,then the ball can move in the electricfield with a constant radius.It is also clear that when the surface tension vanishes,σ=0,the ball collapses with zero radius,a=0.The surface tension on the superconducting ball is caused by the Josephson tunneling along the surface of the ball which means that the c axis is tangential to the radius so thatσ′=Jc o(16)where c o is the unit cell dimension along the c axis and the Josephson coupling energy isH′=−J cosθ(17)whereθ=θ1−θ2−(2e/hc) A.d l(18) is the phase factor.All the grains are aligned in such a way that the c axis is always on the surface.Keeping the c axis on the surface can be achieved by rotations along the a or b axis which are equivalent.Therefore the superconducting ball develops a texture.It was found by us[6]that the normal effects can be changed to superconducting proper-ties by introducing the factor of l s/ξwhere l s is the mean free path of normal electrons andξis the coherence length.The superconducting ball gets charged by the electrodes which is a normal effect.Therefore,due to this normal charge the superconducting Josephson current is reduced by the factor l s/ξ.We suppose that n is the concentration of normal electrons so that the mean free path is l s=(π/3)1/6[a o/(4n1/3)]1/2with a o=¯h2/me2as the Bohr radius. Hence,we can write the surface tension on the surface of the charged ball as,σ=Jc o l s/ξ,(19) which replaces(16).It is sufficient for the present purpose to write J≈J o≈π∆/2SR N where S is the surface area and R N is the normal resistivity and∆=∆o[1−T/T c]1/2so that we can estimate the temperature dependence of the ball radius from,a=[πc o l s D2ξ2/(3∆¯hωR N)]1/3(20) where we have used(15),(19)and S=4πing the fact thatξdiverges asξ=ξo/(1−T/T c)νwhereν≈0.7is the exponent for the divergence of the coherence length,the above expression can be written as,a=[πc o l s D2ξ2o/(3¯hωR N∆o)]1/3/(1−T/T c)1/6+2ν/3.(21) At T=T c,the coherence length diverges and hence,the ball radius for strong pairing shows strong divergence.Case IV.As noted in(7),for the charge depending on radius we rewrite the eq.(10)asU=4πσa2+γ2E2ǫo a32)E2ǫo](22)which gives results similar to those already discussed.A scanning electron micrograph of the superconducting ball formed by the application of an electricfield on a powder of superconducting material contains small grains dispersed in liquid nitrogen is shown by Tao et al[7].It is quite clear that the ball is not perfectly spherical.If the pairing of electrons is an s-wave type,we would expect the formation of a perfectly spherical ball.Therefore,we think that the gap has d-wave symmetry.In which case the gap in(10)and in subsequent relations should be replaced by∆=∆o cos2ϕ.It is also possible that the gap is of complex nature in which the symmetry changes from d(x2−y2)to d(xy)or from s to d(x2−y2)when temperature or magneticfield is varied. The high temperature phase may have higher symmetry than the low temperature phase as found earlier[8].A detailed study of the effect of the electricfield on superconductors is given in a recent book[9].3.Conclusions.In conclusion,wefind that superconducting powder forms a ball when subjected to an electricfield.The pairing interaction plays an important role while Josephson interaction provides the surface tension.The temperature dependence of the ball radius arises from the divergence in the coherence length.References.[1]K.N.Shrivastava,J.Phys.:Condens.Matter5(1993)L597.[2]J.Mannhart,D.G.Schlom,J.G.Bednorz and K.A.M¨u ller,Phys.Rev.Lett.67(1991) 2099.[3]T.Frey,J.Mannhart,J.G.Bednorz and E.J.Williams,Phys.Rev.B51(1995)3257.[4]L.S.Lingam and K.N.Shrivastava,Mod.Phys.Lett.B10(1996)1123.[5]L.S.Lingam and K.N.Shrivastava,Physica B223(1996)577.[6]K.N.Shrivastava,J.Phys.(Paris)Colloq49(1988)C8-2239.[7]R.Tao,X.Zhang,X.Tang and P.W.Anderson,Phys.Rev.Lett.83(1999)5575.[8]N.M.Krishna and K.N.Shrivastava,Physica B230(1997)939.[9]K.N.Shrivastava,Superconductivity:Elementary Topics,World Scientific Pub.,New Jer-sey,London,Hong Kong,Singapore,(2000).。