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量子光电器件及应用英文Quantum photonic devices and applications.Quantum photonic devices refer to devices that utilize the principles of quantum mechanics to manipulate and control light at the quantum level. These devices often involve the generation, manipulation, and detection of single photons, as well as the entanglement of photons for applications in quantum computing, quantum communication, and quantum cryptography.One important example of a quantum photonic device is the single-photon source, which is crucial for many quantum technologies. These sources are used in quantum key distribution systems, quantum metrology, and quantum information processing. They can be based on various physical platforms such as semiconductor quantum dots, trapped ions, or nonlinear optical processes.Another key area of research and development in quantumphotonic devices is quantum photodetectors, which are capable of detecting individual photons with highefficiency and low noise. These detectors are essential for applications such as quantum communication and quantum imaging.In addition to these foundational devices, there is ongoing research into more advanced quantum photonic devices, including quantum gates, quantum memories, and quantum repeaters. These devices are essential for the realization of large-scale quantum networks and quantum information processing systems.The applications of quantum photonic devices are wide-ranging. In quantum computing, for example, quantum photonic devices are used for the manipulation and storage of quantum information in the form of photons. In quantum communication, quantum photonic devices enable secure transmission of information through the quantum key distribution and quantum teleportation. Quantum photonic devices also have potential applications in high-precision sensing and metrology, as well as in the development ofquantum-enhanced imaging techniques.Overall, quantum photonic devices and their applications represent a rapidly growing and highly interdisciplinary field, with implications for both fundamental science and advanced technologies. As research in this area continues to advance, we can expect to see even more innovative quantum photonic devices and novel applications in the near future.。
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文档下载后可定制修改，请根据实际需要进行调整和使用，谢谢!本店铺为大家提供各种类型的实用资料，如教育随笔、日记赏析、句子摘抄、古诗大全、经典美文、话题作文、工作总结、词语解析、文案摘录、其他资料等等，想了解不同资料格式和写法，敬请关注!Download tips: This document is carefully compiled by this editor. I hope that after you download it, it can help you solve practical problems. The document can be customized and modified after downloading, please adjust and use it according to actual needs, thank you! In addition, this shop provides you with various types of practical materials, such as educational essays, diary appreciation, sentence excerpts, ancient poems, classic articles, topic composition, work summary, word parsing, copy excerpts, other materials and so on, want to know different data formats and writing methods, please pay attention!荧光非闪烁IIVI族半导体核壳量子点的研究量子点技术是近年来研究的焦点之一，这种新型半导体材料拥有独特的光学、电学和物理学性质，广泛应用于生物医药、显示、照明和能源等领域。
第26卷第7期电子元件与材料V ol.26 No.7 2007年7月ELECTRONIC COMPONENTS AND MATERIALS Jul. 2007ITO薄膜厚度和含氧量对其结构与性能的影响辛荣生1，林钰2（1. 郑州大学 材料科学与工程学院，河南 郑州 450052；2. 河南教育学院 化学系，河南 郑州 450014）摘要: 在玻璃衬底上用直流磁控溅射的方法镀制ITO透明半导体膜，采用X射线衍射技术分析了膜层晶体结构与薄膜厚度和氧含量的关系，并测量了薄膜电阻率及透光率分别随膜厚和氧含量的变化情况。
以低氧氩流量比(1/40)并控制膜厚在70 nm以上进行镀膜，获得了结晶性好、电阻率低且透光率高的ITO透明半导体薄膜，所镀制的ITO膜电阻率降到1.8×10–4 Ω·cm，可见光透光率达80 %以上。
关键词:无机非金属材料；ITO膜；氧氩流量比；电阻率；透光率中图分类号: TN304.055 文献标识码:A 文章编号:1001-2028（2007）07-0021-03Influence of thickness and oxygen content of ITO thin films on itsstructure and propertiesXIN Rong-sheng1, LIN Yu2(1. College of Material Science and Engineering, Zhengzhou University, Zhengzhou 450052, China; 2. Department ofChemistry, Henan Education Institute, Zhengzhou 450014, China)Abstract: ITO transparent semiconducting films were deposited onto the glass substrates by DC magnetron sputtering method. The film structure related to film thickness and oxygen content was determined by X-ray diffraction. The resistivity and transmissivity changed with film thickness and oxygen content individually were also measured. The ITO transparent semiconducting films with well crystallinity, low resistivity and high transmissivity were obtained, which deposited by processing of low flow ratio O2/Ar(1/40) and film thickness above 70 nm. The ITO film resistivity is 1.8×10–4 Ω·cm and visible light transmissivity is beyond 80 %.Key words: non-metallic inorganic material; ITO film; ratio O2 /Ar; resistivity; transmissivity氧化铟锡（ITO）透明半导体膜具有一系列独特性能，如电导率高、可见光透光率高、膜层硬度高且耐化学腐蚀，在平面显示器、太阳能电池、电致变色镜、热镜、智能窗和薄膜电池等领域获得广泛的应用[1]。
半导体量子点技术在生物成像中的应用研究随着科技的不断发展，生物成像技术也在不断地创新和提升。
生物成像技术作为一种新兴的研究领域，对于生命科学的研究及推动诊断治疗也起到了极大的帮助。
半导体量子点技术作为一种新型的成像材料，也成为目前最具有潜力的材料之一。
下面我们来探讨一下半导体量子点技术在生物成像中的应用以及研究进展。
一、半导体量子点技术概述半导体量子点是一种具有特殊电子性质和结构的材料，由于其尺寸在5-10nm左右，其内在物理和化学性质与体积宏大的材料不同，能够具有一些非常独特的物理和光学性质。
半导体量子点的研究始于1980年代，至今已经有近四十年的发展历程。
近些年来，半导体量子点技术的研究成果已经颇为丰富，应用广泛，尤其是在材料、生物医学等领域颇有应用前景。
二、半导体量子点技术在生物成像中的应用1、半导体量子点在纳米探针中的应用生物成像中，纳米尺度的探针对于显微镜成像具有非常重要的作用。
由于半导体量子点具有天然的发光能力，其可以将生物样品中的目标区域标记出来，从而提高显微镜的分辨率。
不仅如此，由于半导体量子点具有极高的荧光量子产率，因此，它们将很有希望成为提高分子成像灵敏度的探针之一。
2、半导体量子点在组织成像中的应用半导体量子点在组织成像中的应用受到了极大的关注。
由于半导体量子点的小尺寸以及其特殊的荧光性质，使得它们可以被用来制造高分辨率和高敏感度的成像设备。
在内窥镜成像中，半导体量子点可以作为一种非常有潜力的可见光荧光趋近表征的材料，可以扩大显微镜观察范围，并且可以实现不需要溶胶或成像剂的成像模式。
3、半导体量子点在口腔病学成像中的应用半导体量子点也可以用于口腔病学成像领域。
通过使用半导体量子点荧光探针，科学家可以对口腔细胞进行成像，从而检测有关口腔健康的信息。
因此，半导体量子点在口腔病伤、口腔肿瘤、口腔癌等领域中都具有显著的应用价值。
值得注意的是，半导体量子点在成像过程中具有高明亮度、高分辨率和低自由基产生等特点，可以在口腔病诊断和治疗上提供有效帮助。
量子点具有量子力学的英文回答：Quantum dots exhibit quantum mechanical effects due to their nanoscale dimensions. These effects include:Quantization of energy levels: The energy levels of electrons in quantum dots are discrete, meaning they can only occupy certain specific energies. This is in contrast to the continuous energy levels of electrons in bulk materials.Tunable bandgap: The bandgap of a quantum dot is the energy difference between the valence band and the conduction band. The bandgap of a quantum dot can be tuned by changing the size of the dot. This allows quantum dots to be used in a variety of optoelectronic applications.Enhanced optical properties: Quantum dots have enhanced optical properties, such as high photoluminescenceefficiency and narrow emission spectra. These properties make quantum dots ideal for use in applications such as light-emitting diodes (LEDs), lasers, and solar cells.中文回答：量子点由于其纳米尺度的尺寸而表现出量子力学效应。
第 38 卷第 7 期2023 年 7 月Vol.38 No.7Jul. 2023液晶与显示Chinese Journal of Liquid Crystals and Displays量子点在显示应用中的研究进展林永红，黄文俊，张胡梦圆，刘传标，刘召军*（南方科技大学电子与电气工程系，广东深圳 518055）摘要：量子点因具有量子产率高、吸收范围宽、发光光谱窄、发光波长可调等优异的光电特性，使其在显示中展现出巨大的应用前景。
化学溶液法合成的量子点不仅具有制备工艺简单和成本低廉等优势，而且也可通过多种方式实现高分辨率的显示器件。
量子点优异的电致发光和光致发光特性，使其在显示领域具有重要的研究价值。
电致发光的量子点发光二极管，在材料合成和器件结构的研究都获得了快速的发展，为实现商业化的显示器件提供了必要基础。
利用量子点的光致发光显示器件获得了更广的色域，呈现出了更丰富的视觉效果。
本文从量子点的特性、电致发光和光致发光出发，介绍了量子点在显示中的应用，总结了量子点器件的研究现状，分析了在器件发展中存在的问题。
关键词：量子点；电致发光；光致发光；显示中图分类号：TN383；O482.31 文献标识码：A doi：10.37188/CJLCD.2022-0265Research progress of quantum dots in display applicationsLIN Yong-hong，HUANG Wen-jun，ZHANGHU Meng-yuan，LIU Chuan-biao，LIU Zhao-jun*（Department of Electronic and Electrical Engineering， Southern University of Science and Technology，Shenzhen 518055， China）Abstract： The excellent optoelectronic characteristics of quantum dots， such as high quantum yield， wide absorption range， narrow emission spectrum and adjustable emission wavelength， make them show great application prospects in displays.Quantum dots synthesized by chemical-solution methods not only have the advantages of a simple preparation process and low cost， but also can be used to achieve high-resolution displays in various ways. The excellent electroluminescence and photoluminescence of quantum dots make them play an important role in the research of displays.Electroluminescent quantum-dot light-emitting diodes have achieved a rapid development in the research of material synthesis and device structure， which provides a foundation for the realization of commercial displays. The displays using the photoluminescence of quantum dots have attained a wider color gamut and presented a richer visual effect. This paper introduces the characteristics， electroluminescence and photoluminescence of quantum dots， and their applications in displays， summarizes the research status of quantum-dot devices， and analyzes the existing problems in the 文章编号：1007-2780（2023）07-0851-11收稿日期：2022-11-12；修订日期：2022-12-03.基金项目：广东省基础与应用基础研究基金（No.2021B15113001）；深圳市科技计划项目（No.KQTD20170810110313773，No.JCYJ20190812141803608）Supported by Fundamental and Applied Fundamental Research Fund of Guangdong Province （No.2021B1515130001）；Shenzhen Science and Technology Program （No.KQTD20170810110313773，No.JCYJ20190812141803608）*通信联系人，E-mail： liuzj@第 38 卷液晶与显示development of quantum-dot devices.Key words： quantum dots； electroluminescence； photoluminescence； displays1 引言在科技日新月异的今天，显示设备作为一种信息交换媒介，在现代信息化社会占有越来越重要的地位，无论是最初的阴极射线管（Cathode Ray Tube，CRT）显示器、液晶显示器（Liquid Crystal Display，LCD）和发光二极管（Light-Emitting Diode，LED），还是如今的有机发光二极管（Organic Light-Emitting Diode，OLED）、量子点发光二极管（Quan‑tum Dot Light-Emitting Diode，QLED）、Mini Light-Emitting Diode （Mini-LED）和Micro Light-Emit‑ting Diode （Micro-LED）。
Nitrogen-Doped and CdSe Quantum-Dot-Sensitized Nanocrystalline TiO2Films for Solar Energy Conversion ApplicationsTzarara Lo´pez-Luke,†,‡Abraham Wolcott,†Li-ping Xu,†Shaowei Chen,†Zhenhai Wen,£Jinghong Li,£Elder De La Rosa,§and Jin Z.Zhang*,†Department of Chemistry and Biochemistry,Uni V ersity of California,Santa Cruz,California95064,Instituto de In V estigaciones Metalurgicas,Uni V ersidad Michoacana de San Nicola´s de Hidalgo,Ciudad Uni V ersitaria,Morelia Michoaca´n,58060,Me´xico,Department of Chemistry,Tsinghua Uni V ersity,Beijing,100084,China,and Centro de In V estigaciones en Optica,A.P.1-948,Leon Gto.37160,Me´xicoRecei V ed:September12,2007;In Final Form:No V ember2,2007Nitrogen-doped titanium dioxide(TiO2/N)nanoparticle thin films have been produced by a sol-gel methodwith hexamethylenetetramine(HMT)as the dopant source.The synthesized TiO2/N thin films have beensensitized with CdSe quantum dots(QDs)via a linking molecule,thioglycolic acid(TGA).Optical,morphological,structural,and photocurrent properties of the thin films with and without QD sensitizationhave been characterized by AFM,TEM,XPS,Raman spectroscopy,UV-visible spectroscopy,andphotoelectrochemistry techniques.AFM measurements reveals that films with thicknesses of150and1100nm can be readily prepared,with an average TiO2particle size of100nm.TEM shows a uniform sizedistribution of CdSe QDs utilized in sensitizing the TiO2/N films.Doping of the TiO2crystal lattice by HMTwas confirmed to be0.6-0.8%by XPS.Differences in crystal phase caused by the precursors HMT,nitricacid,and poly(ethylene glycol)(PEG)are elucidated using XRD and Raman spectroscopy.The resultantcrystal phase of TiO2/N varies but is a mixture of anatase,brookite,and rutile phases.UV-visible absorptionspectra show that N doping of TiO2causes a red-shifted absorption into the visible region,with an onsetaround600nm.Nitrogen doping is also responsible for the enhanced photocurrent response of the TiO2/Nnanoparticle films in the visible region relative to undoped TiO2films.In addition,CdSe QDs linked toTiO2/N nanoparticles using TGA were found to significantly increase the photocurrent and power conversionof the films compared to standard TiO2/N films without QD sensitization.The incident photon-to-currentconversion efficiency(IPCE)is6%at400nm for TiO2/N-TGA-CdSe solid-state solar cells and95%forTiO2/N-TGA-CdSe films near300nm in a Na2S electrolyte,which is much higher than that of undopedTiO2with QD sensitization or TiO2/N without QD sensitization.The power conversion efficiency(η)wasfound to be0.84%with a fill factor(FF%)of27.7%with1100nm thick TiO2/N-TGA-CdSe thin films.The results show that combining nitrogen doping with the QD sensitization of TiO2thin films is an effectiveand promising way to enhance the photoresponse in the near-UV and visible region,which is important forpotential photovoltaic(PV)and photoelectrochemical applications.1.IntroductionIn recent years,there is an increasing interest to find sustainable alternative energy(SAE)sources due to the height-ening cost of fossil fuels and the detrimental effects of global climate change.Photovoltaic(PV)cells have received significant attention due to the limitless influx of photons from the sun. Recent market energy analysis is predicting energy parity between conventional energy production and PV costs in cents per kilowatt hour(cents/kW h)in only5-8years.1Silicon solar cells have attained a solar conversion efficiency of20%; however,the manufacturing process is very expensive and involves the use of toxic chemicals inherent in the semiconductor industry.To date,there have been reports of two types of solar cells based on nanostructured semiconductor materials,the Gra¨tzel solar cell,based on dye-sensitized nanoporous semiconductor thin films,2and the quantum dot solar cell,based on composite films of semiconductor nanoparticles(CdSe)and conjugated polymers studied by Greenham et al.3Gra¨tzel’s initial report in 1991presented a nanocrystalline dye-sensitized solar cell converting light to electrical energy with an efficiency of7%. The dye-sensitized solar cell(DSSC)consists of TiO2nano-particles acting as a highly porous,wide band gap semiconduc-tor,electron acceptor layer.In the DSSC,visible-light-absorbing dye molecules adsorbed onto the TiO2surface act as the sensitizer to harvest more of the solar flux.Under irradiation, the photoexcited dye molecules inject electrons into the TiO2 layer that are transported through the porous TiO2layer and collected by a conductive fluorine-doped SnO2layer on the glass surface.The oxidized dye is regenerated by a liquid electrolyte, with the highest reported efficiency of about10%.Greenham investigated the processes of charge separation and transport in the interface between a conjugated polymer and semiconduc-tor nanocrystal as a hybrid organic-inorganic system.A quantum efficiency of up12%has been reported with a high*To whom correspondence should be addressed.E-mail:zhang@.Phone:831-459-3776.†University of California.‡Universidad Michoacana de San Nicola´s de Hidalgo.£Tsinghua University.§Centro de Investigaciones en Optica.1282J.Phys.Chem.C2008,112,1282-129210.1021/jp077345p CCC:$40.75©2008American Chemical SocietyPublished on Web01/04/2008concentration of nanocrystals,where both nanocrystals and polymers provide continuous pathways to the electrodes.3The absorption,charge separation,and transport properties of the composites were found to be a function of the size,material, and the surface ligands of the nanocrystals utilized.TiO2and ZnO nanoparticles represent good examples of nanocrystalline materials used for potentially low-cost PV devices for energy conversion as an alternative to silicon solar cell technology4-9and for photocatalysis.10,11Titanium dioxide (TiO2),or titania,exists in three crystalline phases,anatase, rutile,and brookite.Anatase and rutile have found uses mainly in PV cells,photoelectrochemical cells(PEC),and photo-catalysis applications.12-15On the contrary,the brookite phase has not received similar attention,perhaps because it is the most difficult to prepare in the form of a thin film.16Titania has a wide band gap(3.2eV)and absorbs only5%of the solar spectrum,resulting in poor conversion efficiency in solar cell applications.Nonmetal-doped TiO2nanoparticles and nanotubes have been shown to produce electronic states in the TiO2band gap,thereby extending photoresponse to the visible region and improving photoactivity.17-19Recent studies of TiO2/N and ZnO/N have found success in narrowing the band gap and increasing light harvesting efficiency with nitrogen doping,20-22 and further investigations have been focusing on the resulting photoelectrochemical properties and photocatalytic activity for SAE production.Several different synthesis protocols have been developed to produce TiO2/N.The usual doping process involves using ammonia as a nitrogen source by sol-gel,thermal,or hydrothermal chemical methods.23,24An alternative method to obtain TiO2/N involves using HMT by chemical and mechanochemical processes.The resulting effect on crystalline phase composition showed increased photocatalytic activity and photovoltaic properties,with specific morphologies produced.25,26Nitrogen doping within TiO2can be interstitial or substitutional,with the latter being more effective,resulting in mixing of N2p states with O2p states and contributing to the band gap narrowing.27,28Different techniques have been used to study N-doped TiO2crystallo-graphically,including XPS,EPR,Raman spectroscopy,XRD, and absorption spectroscopy.29-31Theoretical studies have supported the visible absorption and the resulting yellowish color of TiO2/N thin films and powders.32,33Alternative techniques to increase the photoresponse besides doping include the utilization of tunable narrow band gap semiconductor nanoparticles or quantum dots(QDs)such as CdS,CdSe,and CdTe to sensitize wide band gap semiconduc-tors such as the metal oxides,for example,TiO2and ZnO.34-37 QDs with their large extinction coefficient strongly absorb visible light,inject electrons into the conduction band of metal oxides,and thereby,contribute to increased solar energy conversion.Attachment of CdSe QDs to nanocrystalline TiO2 has been shown to be successful with an immersion method using a bimolecular linker.38Sonochemical,photodeposition, and chemical bath deposition of CdSe on TiO2nanoparticles and nanotubes has also been studied for photocatalysis applications.39-41However,the use of QDs to improve TiO2-based solar cell efficiency is still an area of active exploration. In this work,we demonstrate a relatively simple approach to dope TiO2nanoparticles with nitrogen and also efficiently sensitize the doped nanoparticles with CdSe QDs.The nano-particle films have been characterized in terms of their structural, optical,and morphological properties using a combination of experimental techniques.The results show substantially en-hanced photoresponse and improved conversion efficiency of the TiO2nanoparticle films when nitrogen doping and QD sensitization are used in unison.Possible explanations are provided in terms of the morphological and optical properties of the films.This method based on combined doping and QD sensitization is promising for solid-state PV cells and photo-electrochemical applications.2.Experimental Section2.1.Sample Preparation.Materials.Titanium(IV)isopro-poxide(#377996,99%),technical-grade trioctylphosphine(TOP-#117854,90%),trioctylphosphine oxide(TOPO-#223301,99%), potassium chloride(KCl-#204099,99%),poly(ethylene glycol) (PEG-#25322-68-3,average M n of ca.10000g/mol),and sodium sulfide(Na2S-#407410,99%)were obtained from Sigma-Aldrich(Milwaukee,WI).Cadmium oxide(CdO-# 223791000,99%)and selenium powder(Se200mesh-#198070500,99%)were obtained from Acros Organics(Morris Plains,NJ).1-Tetradecylphosphonic acid(TDPA-#4671-75-4,99%)was obtained from PCI synthesis(Newburyport,MA). Nitric acid(2.0N-#LC178502)was purchased from Lab.Chem Inc.(Pittsburgh,PA).Thioglycolic acid(TGA-#103036,98%) was obtained from MP Biomedicals Inc.(Solon,OH).F/SnO2 conductive glass(Tec glass30Ohms)was obtained from Hartford glass(Hartford City,IN),and the Ag/AgCl reference electrodes were from CH Instruments Inc.(Austin,TX).TiO2Film Preparation.Four kinds of TiO2films(TiO2-1, TiO2-2,TiO2-3,and TiO2-4)were made by a sol-gel method. All TiO2films were made using375µL of titanium isopropoxide as a precursor,which was stored in a nitrogen-filled glovebox. For TiO2-1,TiO2-2,TiO2-3,and TiO2-4,titanium isopropoxide was injected into250µL of Milli-Q water and5mL of ethanol within the glovebox.TiO2-1solutions did not contain nitric acid, while TiO2-2solutions contained a dropwise addition of nitric acid until the solution reached a pH∼1.23.TiO2-3solutions are similar to TiO2-2,but0.05g of HMT was added under vigorous stirring.Solution TiO2-4is similar to TiO2-3,with an addition of0.90g of poly(ethylene glycol)(PEG-10,000g/mol) under vigorous stirring,all within an O2-free environment.As a point of clarification,acidification(TiO2-2,TiO2-3,and TiO2-4)was performed outside of the glovebox prior to titanium isopropoxide addition within the glovebox.The sol was then stirred for3days within the glovebox at ambient temperatures (∼25°C).All TiO2films were made by spin coating at2000 rpm for60s onto FTO(SnO2/F)conducting substrates in ambient conditions.The thickness was estimated with the mass, area,and density of the TiO2films and was confirmed with AFM measurements(∼150and∼1100nm thick).The films were annealed at550°C for1.5h with a Leister heat gun in open air conditions.For thin TiO2-1,TiO2-2,TiO2-3,and TiO2-4 films,50,200,100,and50µL of the sol solution was used, respectively.For thick TiO2-1,TiO2-2,TiO2-3,and TiO2-4films, 400,2000,700,and250µL,respectively,of the sol solution was used.The sol solution was put on the conductive glass with 2.2cm2areas.It was necessary to apply different volumes in order to obtain approximately the same film thickness because of the different viscosities of the various solutions.CdSe QD Synthesis.High-quality CdSe QDs were synthesized based upon the protocol of Peng et al.,42wherein CdO is utilized as the Cd precursor and TDPA and TOPO are the ligands and coordinating solvents,respectively.The resulting CdSe nano-crystals were in the strong confinement size regime and were synthesized in normal air-free reaction conditions.The synthesis of the CdSe nanoparticle follows the procedure reported by Robel et al.,wherein0.05g(∼0.39mmol)of CdO,0.3g(∼1.1N-Doped and CdSe QD-Sensitized Nanocrystalline TiO2J.Phys.Chem.C,Vol.112,No.4,20081283mmol)of TDPA,and4g of TOPO were heated to110°C and degassed under vacuum and then heated to300°C under a nitrogen flow(Schlenk line).38A SeTOP(0.7%by weight) solution was obtained by adding0.026g of Se powder with 4.25mL of TOP inside of a glovebox and stirring for1h to ensure complete dissolution of the Se powder.After reaching 300°C,the Cd-TDPA-TOPO solution was cooled to270°C prior to the injection of SeTOP.Under a nitrogen flow,3mL of SeTOP was injected,which resulted in the lowering of the temperature to260°C.The temperature was then increased to 280°C to facilitate particle growth,and aliquots were removed and probed to track nanocrystallite growth via UV-vis absorp-tion spectroscopy and photoluminescence(PL)spectroscopy. The CdSe solution was cooled and was removed from the reaction flask at around80°C and dissolved into∼10mL of toluene.The QDs in toluene were then cleaned twice through a precipitation and decantation regime using methanol and centrifugation at3000rpm,and the QDs were ultimately redissolved in toluene prior to their use as a sensitizer. CdSe QD Linkage to TiO2.CdSe QDs were linked to nanocrystalline TiO2and TiO2/N thin films using TGA as a molecular linker.TiO2has a strong affinity for the carboxylate group of the linker molecules,while the sulfur atom of TGA binds strongly to CdSe nanoparticles through surface Cd2+ cations.The films were dried with a heat gun at100°C for4 h to remove H2O from the surface due to ambient humidity adsorption.They were later immersed in undiluted neat TGA for12h in a nitrogen environment in a glovebox.The films were then immersed in toluene,removing the excess TGA,and in turn immersed in a CdSe solution for12h inside of the glovebox.Four films of2.2cm2were immersed in5mL of CdSe QDs suspended in toluene,as described earlier.The TiO2/ N-TGA-CdSe films were stored in a nitrogen-filled glovebox and not exposed to light prior to PEC characterization.The solid-state TiO2/N-TGA-CdSe cell after PEC measurements re-mained stable for months.The TiO2/N-TGA-CdSe cell in the electrolyte is very stable;however,when it is removed from the Na2S electrolyte and are in ambient conditions,the thin films’properties deteriorate after experimentation.Long-term stability needs to be further studied in future research.2.2.Structural and Morphological Characterization.X-ray photoelectron spectroscopy(XPS)studies of the films were carried out on an X-ray photoelectron spectrometer(XPS,PHI Quantera SXM)using a non-monochromatized Al KR X-ray source(1486.6eV).The energy resolution of the spectrometer was set at0.5eV.The binding energy was calibrated using a C 1s(284.6eV)spectrum of a hydrocarbon that remained in the XPS analysis chamber as a calibration tool.Crystalline phase identification was performed via X-ray diffraction(XRD)in conjunction with Raman spectroscopy.XRD analysis was conducted on a MINIFLEX diffractometer operating at30 kV/15mA using Cu-K R radiation and a scanning speed of1°2θ/min.Raman spectroscopy of the films was performed using a Renishaw micro-Raman setup with a(5/10/20/50)×objective lens and a633nm wavelength laser(Research Electro-Optics, Inc.,Boulder,CO).Renishaw’s WiRE(Windows-based Raman environment)was used for collection and data analysis of1-5 scans ranging in accumulations of1-10s.AFM images of the films without and with QDs were acquired under ambient conditions with a PicoLE SPM instru-ment(Molecular Imaging)in tapping mode.The tapping mode cantilevers exhibit resonant frequencies between60and90kHz (typical75kHz),force constants of2.5-5.5N/m,and tip apex radii of∼10nm.The resulting images were flattened and plane-fit using software from Molecular Imaging.Silica-etched tips were purchased from MikroMasch(Watsonville,OR).A JEOL model JEM-1200EX microscope was used for the low-resolution transmission electron microscope(TEM)studies of the CdSe QDs.The TEM was equipped with a Gatan Model 792Bioscan digital camera running on a Windows2000 computer with the Gatan Digital micrograph as the analyzing software.2.3.Optical and Electrochemical Characterization.UV-vis absorption spectroscopy was conducted on a Hewlett-Packard8452A diode array spectrophotometer.UV-vis ab-sorption spectra were measured first by placing a blank FTO glass substrate in the light path,subtracting the absorption pattern,and then performing the UV-vis absorption measure-ment on the collection of TiO2thin films. Photoluminescence(PL)spectroscopy was gathered on a Perkin-Elmer LS50B with an excitation wavelength of390nm and a1%attenuator.QDs in toluene were placed in an open-sided1cm path length quartz cuvette for both UV-vis absorption and PL measurements.Thin films were placed in a thin film sample holder from Perkin-Elmer(#52123130)for PL spectra.Photoelectrochemical studies(linear sweep voltammetry and incident photon-to-current conversion efficiency(IPCE))in the solid state and in an electrolyte were carried out with a CHI440 electrochemical workstation(Austin,TX).Linear sweep vol-tammetry was used to obtain the I-V profiles.Actively investigated thin films were the TiO2,TiO2/N,and TiO2/ N-CdSe thin films described earlier.A Ag/AgCl and Pt wire coil were used as the reference and counter electrodes, respectively.Before each measurement,the Na2S electrolyte solution was deaerated by bubbling ultrahigh-purity N2for20 min through the electrolyte solution,and a nitrogen flow was also subsequently blown over the surface during data gathering. For I-V measurements,a halogen lamp was utilized(75W), and for IPCE measurements,a1000W Xe lamp(Oriel Research arc lamp assembly#69924and power supply#69920)coupled to an infrared(IR)water-filled filter(Oriel#6127)was used and then aligned into a monochromator(Oriel Cornerstone130 1/8m)for spectral resolution from300to800nm.An aqueous Na2S solution served as the redox couple to maintain the stability of the QDs,as discussed elsewhere.43,44A maximum photo-current was produced with1M Na2S using6.5mL of Milli-Q water(18MΩ).Diminished molarities of Na2S were found to decrease the overall photocurrent performance.IPCE measure-ments were also conducted for solid-state TiO2/N-TGA-CdSe solar cells.A schematic of the photoelectrochemical setup is shown in Figure1a,b.3.ResultsThe amount of nitrogen in N-doped TiO2nanoparticle films was identified by the XPS technique.Figure2shows the XPS spectra for the TiO2-3film.Figure2a shows the Ti2p,O1s,C 1s,N1s,and Sn binding energies from0to1000eV(Sn is identified from the conductive film).Figure2b shows only the nitrogen binding energy from396to408eV,showing two peaks at400and above401.2eV.To compare the effect of HMT and PEG,the TiO2-3film has37.45%of C,0.60%of N,50.16% of O,and11.79%of Ti.The TiO2-4film shows elemental compositions of C and N of39.93and0.80%,respectively,with 48%of O and11.27%of Ti.Figure3shows the XRD patterns for all of the films prepared for determining the crystal phases of both TiO2and TiO2/N.1284J.Phys.Chem.C,Vol.112,No.4,2008Lo´pez-Luke et al.All of the XRD data show the crystal phase of the conductive glass (SnO 2/F),which is marked as SnF.The TiO 2-1film (without nitric acid)XRD pattern is representative of the anatase crystal phase (Figure 3a).For the TiO 2-2film (with nitric acid),the brookite and rutile phases appear,with a mix of anatase,brookite,and rutile phases (Figure 3b).With HMT added for the TiO 2-3film,a mixture of brookite and rutile phases dominates,with a trace amount of the anatase phase (Figure 3c).With further addition of PEG and HMT,as in the TiO 2-4film,the brookite and rutile phases decrease,and the anatase is predominant.Additional evidence for the various crystal phases of the different films comes from their Raman spectra that show anatase as the primary phase for TiO 2-1and TiO 2-4(Figure 4a and c)and a mix of anatase,brookite,and rutile phases in TiO 2-2and TiO 2-3films.Representative Raman spectrum of the TiO 2-3film is shown in Figure 4b.It can be seen that the characteristic peak for the anatase phase at 144cm -1shifts slightly to 146cm -1with nitrogen doping (TiO 2-3and TiO 2-4films).The morphology of the films with and without QDs was studied by AFM in ambient conditions.The average TiO 2nanocrystal size was found to be around 100nm in diameter.Films with HMT (TiO 2-3)or HMT plus PEG (TiO 2-4)show more porosity than TiO 2-1and TiO 2-2films.A representative AFM image for the TiO 2-3film with about 150nm thickness is shown in Figure 5a.For thicker films (e.g.,1100nm),TiO 2particles were observed to form clusters.All films showed the presence of CdSe QDs after they were sensitized,as exemplified by the AFM image for the TiO 2-3-TGA -CdSe film shown in Figure 5b.While it is not easy to determine the exact size of the CdSe QDs based on AFM,the average size of the QDs appear to be on the order of a few nanometers.To better char-acterize the CdSe QDs,TEM measurements were conducted.Figure 6shows a representative TEM image of CdSe QDs.The image appears to show reasonably uniform size distributions of the CdSe QDs,with an average diameter of 3.5nm.Optical absorption of the different films was characterized by UV -vis spectroscopy,with emphasis on comparing the effect of nitrogen doping through HMT or nitric acid.Figure 7shows a comparison of the UV -vis absorption spectra of different films.For films without HMT and PEG (TiO 2-1and TiO 2-2),the absorption is primarily around 340and 400nm (Figure 7a and b).However,for films with HMT (TiO 2-3and TiO 2-4),the spectra show an obvious red shift of the absorption edge toward the visible region,with peaks around 350,426,and 542nm (Figure 7c)or 412and 532nm and an absorption onset at 600nm (Figure 7d).Figure 7e shows the UV -vis absorption spectrum of the TiO 2-3-CdSe film,with strong absorption around 560nm due to the CdSe QDs.For comparison,Figure 8a and b shows the absorption and photoluminescence (PL)spectra of CdSe QDs in toluene under ambient conditions.The absorption spectrum shows the expected strong and sharp excitonic peak around 560nm,while the PL spectrum showsaFigure 1.Photovoltaic schematic of TiO 2/N -TGA -CdSe cells in a (a)Na 2S electrolyte and a (b)solid.(c)Representation of TiO 2-3and TiO 2-4nanoparticles functionalized with CdSe linked with a bifunctional molecule TGA in a porous film (using HMT and HMT +PEG in the TiO 2synthesis,respectively).(d)Representation of a TiO 2-1-TGA -CdSe film (without nitric acid)and a TiO 2-2-TGA -CdSe film (using nitric acid in the TiO 2synthesis)with low porosity.N-Doped and CdSe QD-Sensitized Nanocrystalline TiO 2J.Phys.Chem.C,Vol.112,No.4,20081285narrow emission band near 580nm,which is clearly due to band edge emission.Figure 8c shows the PL spectrum of the TiO 2-3-TGA -CdSe film,with a relatively weak emission peak at 575.5nm,which is slightly blue-shifted with respect to the PL peak of CdSe QDs in toluene solution.The current -voltage (I -V)profiles for solar cells fabricated using films with different thicknesses (150and 1100nm)were obtained using a halogen lamp and 1M Na 2S.With the cell configuration shown schematically in Figure 1,the I -V pro-files measured are shown in Figure 9.The cell without HMT (TiO 2-1-CdSe film)presents a low short-circuit current density of -169µA/cm 2with an open-circuit voltage of -1.3V (Figure 9a)within the voltage window of -1.4-0.3V,and the fill factor is 14.8%with a low power conversion efficiency of η)0.120%.For the cell based on the TiO 2-3-TGA -CdSe thin film (150nm),the I -V curve in Figure 9b shows a short-circuit current (-242µA/cm 2)with an open-circuit voltage of -1.0V,a fill factor of 25.4%,and a power efficiency of η)0.228%.The cell based on the TiO 2-3-TGA -CdSe thick film (1100nm)has the highest short-circuit (-683µA/cm 2)with an open-circuit voltage of 1.2V (Figure 9c)and the highest fill factor of 27.7%and a power conversion efficiency of η)0.840%.The short-circuit current and open-circuit voltage found in Figure 9are summarized in Table 1.The fill factor (FF)and power conversion efficiency (η%)were calculated using the short-circuit current and open-circuit voltage 45and are also given in Table 1where j sc is the short-circuit current density,V oc is the open-circuit voltage,jV max is the maximum power observed from the current density -voltage curve for each device,and I i is the incident light power density (27mW/cm 2).It is clear that thick films (∼1100nm)exhibit a higher FF and η%than thin films (∼150nm).However,cells with TiO 2/N nanoparticles sensitized with CdSe QDs exhibit a much higher η%than films without sensitization.The incident photon-to-current conversion efficiency (IPCE)was studied for solid cells and in an electrolyte (1M Na 2S)with different thicknesses.The IPCE at different wavelengths was determined from the short-circuit photocurrent (j sc ),where V )0at different excitation wavelengths (λ)using the expressionwhere I inc is the incident light power.The IPCE results of TiO2Figure 2.(a)XPS spectra of a nitrogen-doped TiO 2-3film on a SnO 2/F substrate showing C,N,Ti,O,and Sn spectra peaks.(b)A detailed N 1s XPSband.Figure 3.XRD patterns of TiO 2films on the SnO 2/F substrate prepared by the sol -gel method,using different chemicals in the TiO 2synthesis,annealed at 550°C for 1.5h with a heat gun for (a)a TiO 2-1film (without nitric acid),(b)a TiO 2-2film (using nitric acid),(c)a TiO 2-3film (using nitric acid and HMT),and (d)TiO 2-4(using nitric acid,HMT and PEG).The phases found in the films are anatase,rutile,and brookite,marked by A,R,and B,respectively.SnF represents the crystal phase of the conductive glass (SnO 2/F).Figure 4.TiO 2film Raman spectra of (a)TiO 2-1(without nitricacid,HMT,or PEG),(b)TiO 2-3(using nitric acid and HMT in the synthesis),and (c)TiO 2-4(using nitric acid,HMT,and PEG in the synthesis).FF )(jV max )/(j sc V oc )(1)η)(jV max )/I i )FF(j sc V oc )/I i(2)IPCE%)[(1240×j sc (A /cm 2)]/[λ(nm)×I i (w /cm 2)]×100(3)1286J.Phys.Chem.C,Vol.112,No.4,2008Lo ´pez-Luke et al.and TiO 2/N without and with CdSe QDs are shown in Figure 10.It is clear that the photocurrent response is much stronger with the presence of CdSe QD sensitization.The TiO 2-1-TGA -CdSe film (QD-sensitized but without N doping)shows photocurrent responses at 300,530,and 620nm,with the highest response near 300nm with IPCE )56%.The TiO 2-3-TGA -CdSe film (QD-sensitized and N-doped)shows similar response but an overall stronger IPCE with the highest response around 95%at 300nm.The IPCEs of both films closely match the absorption spectrum of CdSe QDs and TiO 2/N,as shown in Figure 7e.The IPCE%is 1.06at 300nm for TiO 2/N (magnified 10×in Figure 10),showing photocurrent responses at 320,360,and 420nm,which are also close to the absorption spectrum as shown in Figure 7c.In addition,the IPCE%of TiO 2-3-TGA -CdSe thin film (150nm)solid cell was studied,and the result is shown in Figure 11.It can be seen that peaks at 480and 600nm correspond to absorption of the CdSe QDs (see Figure 11inset that shows the absorption and emission spectra of the CdSe nanoparticles in toluene).This cell shows an IPCE response around 6%at 400nm,and the results are summarized in Table 2.4.Discussion4.1.N Doping of TiO 2Nanoparticles with HMT.It is known that HMT hydrolyzes in aqueous solutions toformFigure 5.AFM images of (left)a nitrogen-doped TiO 2-3thin film (175nm)and (right)N-doped TiO 2-3nanoparticles linked to TGA -CdSe nanoparticles (film thickness ∼1100nm).Figure 6.Representative TEM image of CdSe quantum dots showing an average particle size around 3.5nm.Figure 7.UV -vis absorption spectra of the films (a)TiO 2-1(without nitric acid),(b)TiO 2-2(using nitric acid in the synthesis without HMT),(c)TiO 2-3(synthesized with nitric acid and HMT),(d)TiO 2-4(synthesized with HMT and PEG),and (e)TiO 2-3-TGA -CdSe.The blank was the substrate of SnO 2/F (of the conductive glass).N-Doped and CdSe QD-Sensitized Nanocrystalline TiO 2J.Phys.Chem.C,Vol.112,No.4,20081287。
Quantum Dots for Biomedical ImagingQuantum dots, or QDs, are a type of semiconductor nanocrystals that have unique optical and electronic properties. These properties make them ideal for use in biomedical imaging applications, where they can serve as fluorescent labels to visualize cells, tissues, and organs. In this article, we will discuss the properties of quantum dots, their applications in biomedical imaging, and future prospects for their use in medical research.Properties of Quantum DotsQuantum dots are typically made of semiconductor materials such as cadmium selenide (CdSe) or indium phosphide (InP). They have a small size, usually between 2 and 20 nanometers, which allows them to emit and absorb light at specific wavelengths. Their optical properties are determined by the size and composition of the nanocrystals, which can be tuned to produce a wide range of colors, from blue to red.One of the most important properties of quantum dots is their high quantum yield, which is the efficiency at which they emit light. Unlike organic dyes, which can lose their brightness over time, quantum dots have a long lifetime and can emit light for hours or even days. This makes them ideal for use in imaging applications, where they can be tracked over long periods of time.Another property of quantum dots is their resistance to photobleaching, which is the process of losing fluorescence when exposed to light. Organic dyes can be easily photobleached, which limits their usefulness in imaging applications. Quantum dots, on the other hand, can withstand higher levels of illumination without losing their fluorescence, which allows for more robust imaging.Applications of Quantum Dots in Biomedical ImagingQuantum dots have a wide range of applications in biomedical imaging, including in vivo imaging of cells, tissues, and organs. They can also be used for in vitro imaging of cells and tissues, as well as for tracking the movement of drugs and nanoparticles in living organisms.One of the most important applications of quantum dots is their use in cancer imaging. Because tumors can be difficult to locate using traditional imaging techniques, such as CT scans or MRI, quantum dots can be used to target specific cancer cells and tissues. By conjugating quantum dots with tumor-specific antibodies, researchers can label cancer cells with fluorescent markers that can be visualized using fluorescence microscopy. This can help improve the accuracy of cancer diagnosis and treatment.Quantum dots can also be used to track the movement of drugs and nanoparticles in living organisms. By attaching quantum dots to the surface of drug molecules or nanoparticles, researchers can track their distribution and clearance from the body. This can help improve the delivery and efficacy of drug therapies, as well as reduce the risk of toxicity.Future Prospects for Quantum Dots in Biomedical ImagingThe use of quantum dots in biomedical imaging is still in its early stages, and there is much research to be done to improve their safety, efficacy, and applicability. One of the major concerns with quantum dots is their potential toxicity, as some studies have shown that they can cause damage to cells and tissues.To address these concerns, researchers are developing new types of quantum dots that are less toxic and more biocompatible. These include hybrid quantum dots, which combine the optical and electronic properties of inorganic quantum dots with the biocompatibility of organic molecules. By tweaking the composition and structure of these hybrid quantum dots, researchers hope to improve their safety and effectiveness for use in biomedical imaging.Another area of research is the development of new imaging techniques that can enhance the sensitivity and resolution of quantum dots. These include super-resolution microscopy, which allows researchers to visualize structures and cells at the nanoscale level, and multimodal imaging, which combines multiple imaging modalities to provide a more comprehensive view of biological tissues and structures.ConclusionQuantum dots are a promising new technology for use in biomedical imaging, with the potential to revolutionize the way we diagnose and treat diseases. Their unique optical and electronic properties make them ideal for use as fluorescent labels in imaging applications, allowing researchers to visualize cells, tissues, and organs with greater accuracy and precision. While the use of quantum dots in biomedical imaging is still in its early stages, ongoing research and development promise to improve their safety, efficacy, and applicability in medical research and clinical practice.。
a r X i v :c o n d -m a t /0008277v 1 [c o n d -m a t .s t a t -m e c h ] 18 A u g 2000Quantum Films Adsorbed on Graphite:Third and Fourth Helium LayersMarlon Pierce and Efstratios ManousakisDepartment of Physics and Center for Materials Research and Technology,Florida State University,Tallahassee,FL32306-4350(February 1,2008)Using a path-integral Monte Carlo method for simulating superfluid quantum films,we investigate helium layers adsorbed on a substrate consisting of graphite plus two solid helium layers.The solid helium layers are modeled first as inert,with paths frozen at equilibrated positions,and then as active,with second layer atoms included in the Monte Carlo updating.In both cases,we observe the formation of as many as three well defined additional layers above the first two,and determine the layer promotion density by calculating the density profile and through a calculation of the chemical potential.For liquid layers adsorbed onto the inert solids,we find self-bound liquid phases in both the third and fourth layers and determine the equilibrium density.In the third layer at coverages below equilibrium,we find liquid droplets and a metastable uniform liquid phase,and determine the spinodal point that separates these regions.The above phases and their coverage ranges are in good agreement with several experiments.The superfluid density as a function of coverage is also calculated,and it is observed to change only weakly around the promotion density.For coverages above the beginning of fourth layer promotion,we observe continued increase in the third layer density.We note that the third layer density increase is perhaps enough to cause solidification in this layer,which would explain heat capacity peaks observed experimentally for fourth layer coverages and would provide a simple explanation for the plateaus seen in the superfluid coverage.For helium adsorbed on an active second layer,we observe that a self-bound liquid phase occurs in the third layer,and we determine the equilibrium density and spinodal point,which remain in agreement with experiment.We find that promotion to both the third and fourth layers is signaled by a change in the density dependence of the chemical potential.We further observe the increase in the second layer density with increasing total coverage.The coverage dependence of the superfluid density is calculated,and a pronounced drop is seen at high third layer coverages,as has also been observed experimentally.I.INTRODUCTIONHelium films adsorbed on graphite exhibit a number of phases and have proven to be a rich source for both experimental and theoretical studies of two-dimensional phenomena.The graphite substrate is ordered on atomic length scales and offers a potential well for helium that is relatively strong for physical adsorption but is short ranged perpendicular to the substrate.As a result,a number of distinct,atomically thin layers occur,each with its own phase diagram.Near the graphite surface,the layers tend to solidify,with both commensurate and incommensurate solids occuring in the first two layers.In both layers,this solidification occurs before promotion to the next layer.The second layer exhibits a coverage re-gion with superfluidity as well,1–4while the first layer apparently favors the formation of solid clusters over liq-uid droplets at low densities,5–7a lthough there is debate on this issue.8,9A general review of physically adsorbed films such as helium on graphite can be found in the book of Bruch,Cole,and Zaremba.10In this paper,we will focus on the liquid third and fourth layers.Detailed information on the film structure at low tem-peratures for the third and higher layers has come from a number of experiments,including heat capacity,11,8tor-sional oscillator,1,2,12and third sound measurements.13Ranges in which the heat capacity depends linearly on the coverage suggest that gas-liquid coexistence regions exist in the third and fourth layers.The transition from liquid droplets to a uniform liquid phase in each layer is signaled by a peak in the isothermal compressibility.13Unlike the first and second layers,these higher layers do not solidify before layer promotion,since torsional oscil-lator measurements detect superfluidity for all coverages beginning at intermediate third layer densities.This ap-parently rules out earlier suggestions that the third layer may solidify,8,13although it still may be possible for solid-ificatino to take place under compression of higher layers.Perhaps the most unusual feature of the higher layers is the step-like behavior of superfluidity with increasing density.1,2The superfluid coverage in layered films will not grow continuously because of the layering transitions.Plateaus in the superfluid density immediately after layer promotion are expected,and have been observed,when the particles are in the droplet region.These plateaus occur because the droplets in the new layer lack the con-nectivity to exhibit superflow across the entire surface.14The interesting observation is that the plateaus actually begin before layer promotion for the third through sixth layers.At 500mK,superfluidity even exhibits a decrease with increasing coverage near the promotion to the fourth layer.These effects have been discussed in the context of the Bose-Hubbard model.15The suggestion is that theplateaus are produced by the increased localization of particles in the dense liquid.Theoretical tools applied in the study of quantum films on realistically treated substrates include the hy-pernetted chain Euler-Lagrange(HNC-EL)method,16–19 density functional theory,20–22and quantum Monte Carlo.23–26,9,3,4The HNC-EL approach has been used extensively in studies of the third and higher layers of heliumfilms on graphite.This approach determines sta-ble coverages of the helium layers.Because the theory requires a uniformfilm,calculations cannot be made for all coverages.Breakdowns in the theory are interpreted as occuring at coverages where thefilm is unstable to the formation of droplet patches or to layer promotion.The theory predicts at least three liquid layers will form on top of the solidfirst and second layers,and yields a max-imum coverage value before promotion of0.065atom/˚A2 for each layer,in good agreement with,but somewhat below,the experimental value of0.076.8,13In this paper we present results for the third and fourth helium layers using the path-integral Monte Carlo (PIMC)method.Our simulation is able to take into ac-count the effect of the second layer’s corrugations and zero-point motion on the third layer.We are also able to allow for the possibility of promotion and demotion of particles between the second,third,and fourth layers, and these effects are observed.Finally,our simulation method can be applied to the entire range of possible phases in a layer,from liquid droplets to full solidifica-tion.Thus we are able to probe both the low density and high density phases of a layer.———————————————————————–II.SIMULATION METHODOur simulation is performed using a path-integral Monte Carlo method that includes particle permutations and substrate effects.The general method for bulk sim-ulation has been reviewed elsewhere,27and our modi-fications for simulating layered systems are given in a previous publication.4Some of the present calculations have required minor changes to this method,so we will briefly outline the procedure now in order to explain the modifications.A.Overview of procedureThe partition function Z for a system of N bosons at the inverse temperatureβcan be expanded as a path-integral by inserting M intermediate configurations: 1Z=frozen and are no longer included in the sampling.Ad-ditional atoms are then placed above this inert substrate and have their positions and permutations sampled.We refer to these additional atoms as being“active”.The bisection level used in this simulation was l=3.All er-rors that we report for these calculations are statistical errors arising from the Monte Carlo simulation of the ac-tive atoms above a particular frozen second layer.There will be additional systematic errors that arise from our particular choice for the frozen second layer configura-tion.In the second set of calculations,both the second and third layer particles are included in the sampling.There is a potential problem with doing Monte Carlo calcu-lations on such a system.The third layer is liquid and permutations will occur at low temperatures.This favors using l=3for this layer.The second layer,on the other hand,is solid and furthermore increases in coverage as the overall coverage is ing l=3produces a low acceptance rate for particle moves in the compressed second layer.Sampling efficiency can be improved by us-ing l=2for this layer.Thus a single value of l for the entire system is not optimum.To give each layer the best value of l,we have parti-tioned atoms into“second layer”and“third layer”atoms.“Second layer”atoms have their positions initially taken from an equilibrated second layer solid.“Third layer”atoms are started from an initial configuration of atoms placed at various heights above the second layer.Promo-tion and demotion between the layers are allowed,but the layer label of the atoms does not change.“Third layer”atoms are sampled with l3rd=3,while“second layer”atoms are sampled at l2nd=2.Permutations are allowed between atoms with the same layer label,but we do not allow atoms with different layer labels to permute.This limitation is not a problem for atoms demoted to the sec-ond layer after being initially placed on the third layer, since exchanges are uncommon in the solid second layer. The promotion of a particle from the second layer to the third occasionally occurs after a particularly long simu-lation.At most,only one particle is promoted,so this will have little effect on the permutations in the third layer in all but the lowest coverages.This approach gives a higher acceptance rate for second layer particle moves, while allowing permutations to occur reasonably often in the third layer.We have found that this partitioning of l lowers the energy by an amount ranging from0.0K to0.1K per atom relative to energy calculations with a single value l=3for the entire system.The smallest en-ergy shift occurred at the lowest coverage tested,0.2286 atom/˚A2.At this coverage the two values were within error bars.The greatest energy shift occurred near the equilibrium coverage of the third layer liquid.This was the highest coverage we tested.We also compared the energy values at selected coverages to calculations per-formed with a single value l=2for the entire system and found them to agree,but third layer superfluidity was suppressed,as expected.III.RESULTS FOR THE THIRD AND FOURTHLAYERSBefore presenting our simulation results,we wish to clarify the convention that we use to report adsorbed helium coverages.Normally,the values we give for the density are for the total adsorbed helium(first two solid layers plus any additional coverages for the higher layers). Relative coverages within a layer are prefaced with a ref-erence to that layer.For example,the coverage0.2583 atom/˚A2using our standard simulation cell corresponds to a third layer coverage of0.0466atom/˚A2plus two solid layers with a combined coverage of0.2117atom/˚A2.A.Results with the inert second layeryer promotion and demotionIn order to study multiple layers of the heliumfilm,it isfirst necessary to establish that our simulation does, in fact,produce distinct layers with increasing cover-age.This is illustrated in Fig.1,which shows the growth of the density profile perpendicular to the substrate for the third and higher layers with increasing density.The peaks associated with the third and fourth layers can be clearly observed,as can the beginning of thefifth layer peak at the highest simulated coverage.Also as can be seen in thefigure,the continued growth of the third layer peak for all coverages indicates compression of this layer even after atoms are promoted to the higher layers. Promotion to the fourth layer occurs for coverages greater than0.2837atom/˚A2(17active atoms).This coverage may be determined from the following obser-vation,which is illustrated in Fig.2.For coverages at and just below promotion,increasing the coverage in-creases the peak height but does not appreciably change the profile’s width.Just above this density,the peak height does not change,but an abrupt increase in the width is observed.With increasing coverage,the tail of the profile just above layer promotion evolves into the fourth layer peak.This is also illustrated in thefigure. This value for layer promotion is in agreement with ex-periment.Heat capacity measurements11,8show layer promotion at0.288atom/˚A2,while isothermal compress-ibility measurements13give a somewhat lower value0.280 atom/˚A2for the promotion density.Layer promotion is also signaled by a change in the density dependence of the chemical potential.By differ-encing our calculated total energy values,we can obtain the chemical potentialµ.This is plotted in Fig.3.The values for the energy per particle are given in Table I. As can be seen from thefigure,the chemical potential increases rapidly just below layer promotion until it be-comes favorable to promote an atom to the next layer. Above layer promotion,the chemical potential remains roughly constant with increasing density,as it should for3.08.013.0z(A)0.00.51.0ρ(z )FIG.1.Density profiles at 400mK for the second,third,and fourth layers as a function of height above the graphite substrate.The leftmost peak is for the frozen,equilibrated second layer.The coverages shown begin at 0.2329atom/˚A 2and increase in increments of 0.085,up to 0.3345.The density profile for 0.3811atom/˚A 2is also shown.The profiles are normalized so that integration gives the number of atoms.6.08.010.012.014.0z(A)0.00.10.20.30.40.5ρ(z )FIG.2.Density profiles near layer promotion.The cover-ages are 0.2794,0.2837,0.2879,0.2964,and 0.3005atom/˚A 2.NE/N (K)NE/N (K)0.22860.29640.23290.30060.23710.30480.24130.30910.25830.31330.26250.31750.26670.32180.27100.32600.27520.33020.27940.33450.28370.33870.28790.34290.29210.3472additional demoted atom increases the energy required to further increase the layer density,so µat promotion is only a rough estimate of the chemical potential required to demote additional atoms.Particle demotion may be understood as a balance be-tween many factors.Atoms are initially promoted above a layer because this is energetically favorable.The first promoted atom loses the large energy benefit for being close to the substrate,but gains kinetic energy since it is free to move about on the open surface,with its wave function no longer constrained by the other atoms.It also retains some of the potential energy advantage gained from having neighboring helium atoms.Adding more atoms to the system will favor the formation of droplets.However,once droplets have formed in the new layer,the next atom added to the system faces a different choicethan the first promoted atom.This is illustrated in Fig.5.If it goes into the less dense outer layer,it gains some attraction from the other atoms in this layer.However,it no longer gains the kinetic energy advantage that the first promoted atom had.On the other hand,if the added atom goes into the dense lower layer,it regains the benefit of being closer to the strongly attractive substrate.Fur-thermore,it has the energy advantage for having more helium neighbors:those in its layer,those in the layer above,and those in the layer below (not shown).In con-trast,if the added atom goes into the outer layer,it has less neighbors.It should be noted that each demotion to the dense layer does significantly increase the chemical potential for this layer,so at some point it will be more favorable for atoms to be added to the outer layer again.Coverage(A −2)−18−16−14−12−10−8C h e m i c a l P o t e n t i a l (K )FIG.3.Chemical potential for third and fourth layers.The arrows indicate the third layer promotion density (P)and the fourth layer equilibrium coverage (E).The densities denoted by “D”and the dark arrows are discussed in the text.z(A)00.20.40.6ρ(z )FIG.4.Density profiles for the third and fourth layers.Coverages are given in ˚A −2.(B)(A)demotion.The shaded atom is added to the layered system.In (A),it joins the less dense outer layer.In (B),it is demoted to the more dense underlayer.2.Third and fourth layer phasesWe expect four principal regions in the third layer be-fore fourth layer promotion.These are a low density gas phase (which will have a negligible density at low temper-atures),a droplet region,a metastable liquid region,and an equilibrium liquid phase.The droplet region consists of a liquid phase separated from a low-density gas by an interface.In the metastable region,the droplet phase is replaced by a stretched uniform phase which has nega-tive spreading pressure.The crossover from the droplet region to the stretched liquid phase occurs at the spin-odal point.Direct evidence that the layer is liquid comes from torsional oscillator measurements,which detect su-perfluidity up to layer completion.The isothermal com-pressibility has been measured for the third and higher layers 13and exhibits a divergence that is associated with the spinodal point.The equilibrium liquid coverage can be inferred from heat capacity measurements.11Below,we present evidence of each of these phases using several different observables.First,we can establish the existence of droplets and a uniform liquid phase at different densities with the ra-dial distribution function,g(r),which provides a direct probe of short and long range behavior.Calculations for the third layer are shown in Fig.6.These are plotted as functions of the magnitude of the distance vector be-tween pairs of atoms,projected onto the plane of the sub-strate.These calculations of the averaged g (r )smooth out possible anisotropies induced by the corrugations of the underlying solid helium layer.The g(r)for the three coverages shown in Fig.6are representative of the droplet region,the equilibrium liquid,and the liquid near layer promotion.At the lowest coverage,0.233atom/˚A 2(5active atoms),the radial distribution function drops be-low unity at large distances,as would be expected for a droplet phase.The actual dimensions of the droplet for a given density depend on the size of the simulation cell.For the intermediate coverage,0.2624atom/˚A 2(12active atoms),the first peak has changed only slightly,but the long range behavior is noticeably different,rising again past unity instead of dropping continuously.At the high-est coverage,0.284atom/˚A 2,the system shows evidence of increased correlation,but the long range behavior can-not be determined due to the small size of the simulation cell.This largest coverage also has a different short range behavior,showing an increased probability that the pro-jected distance between two atoms will be less than 2.0˚A .In part,this is a result of the thickening of the layer,as can be seen in Fig.1.Because the atoms can be at different heights above the substrate,the projected dis-tance between them can become smaller than would be possible in strictly two-dimensional calculations.We can also gain some insight into the layer phases by examining contour plots of the probability distribution of atoms in the plane of the substrate.Plots near equi-librium and layer promotion are shown in Fig.7.Ther(A)0.00.51.01.5g (r )FIG.6.The radial distribution function for the third layer at the indicated coverages,in atom/˚A 2.high density liquid shows noticeably more localization than the equilibrium fluid.An increased correlation can also be seen in the radial distribution function at high density.This suggests that the system may be nearing a liquid-solid coexistence phase.FIG.7.Probability densities for the third layer liquid near equilibrium (0.2625atom/˚A 2,left)and just before layerpromotion (0.2837atom/˚A 2,right).Having established that the third layer has gas-liquidand uniform liquid phases,we can next determine the equilibrium coverage ranges of these phases.This can be done by using the Maxwell construction.At low tem-peratures,the total energy and the total free energy are nearly equal,so coexistence regions may be identified by applying the Maxwell construction to the energy.The results for the low temperature (400mK)temperature scans are shown in Fig.8.We have verified that the en-ergy values shown are effectively zero temperature results by recalculating some values at 500mK.In all cases the calculations at the two temperatures agreed within error bars.The values shown in the figure have been shifted by the amount N act e 0,where e 0=−15.897±0.024is the minimum energy per particle,and N act is the number of active atoms in the simulation.At low temperatures,the gas phase will have zero coverage and thus zero to-tal energy.We can thus draw a coexistence line between the beginning of the third layer,0.2117atom/˚A2,and the coverage with minimum energy per particle.This higher coverage is the equilibrium liquid density.We find this coverage to be0.2625atom/˚A2(N act=12). The best chi-squared parabolicfit around this minimum gives0.2645(9)atom/˚A2.The number in parenthesis is the error in the last digit.At this density the layer is com-pletely covered by a uniform liquid.Below this value,the system enters the gas-liquid coexistence region.The en-ergy values in the coexistence region lie above the coexis-tence line,either because the liquid phase is unphysically uniform,or else because of the appreciable cost for creat-ing a phase boundary in afinite-sized system.The third layer equilibrium coverage,0.0528(9)atom/˚A2,is compa-rable to(and slightly higher than)the equilibrium cover-age found for the second layer,0.0480(6)atom/˚A2.3,4For both layers,simulated with the same size cell,the energy minimum occurs when the system contains12atoms. The equilibrium density that we determine is in good agreement with both heat capacity and torsional oscilla-tor measurements.In the measurements of Greywall,11,8 the low temperature heat capacity depends linearly on density from the beginning of the third layer to0.260 atom/˚A2.This linear dependence is a signal of phase coexistence.29The torsional oscillator measurements2 provide evidence of a similar region.The temperature of the oscillator’s dissipation peak,which gives a rough estimate for the superfluid transition temperature,is in-dependent of coverage from0.22to0.26atom/˚A2.This is characteristic of a surface covered by liquid droplets.14 Increasing the coverage in the droplet region increases the size of the droplets,not their density,and so the transition temperature remains constant.For densities above the equilibrium coverage and below layer promotion,we have not been able to rule out the on-set of solid-liquid coexistence in the third layer.Our pro-cedure for identifying such regions in the second layer was to look for the instabilities in the total energy that signal phase coexistence.This was possible because that layer solidified before layer promotion.In the present case, though,the energy cannot be used tofind solid-liquid coexistence because any instabilities that might signal solidification are inextricably entangled with the liquid-vapor phase coexistence that occurs in the fourth layer at the same coverages.Specifically,layer promotion begins at the third layer coverage of0.0720atom/˚A2.The next increment in coverage that we can simulate using the cell described in Sec.II is0.0762atom/˚A2.In our previous simulation of the second layer using the same sized cell,3,4 we determined that an incommensurate solid begins to form at the second layer coverage0.0762atom/˚A2.Ap-parently,promotion preempts solidification.Also of note is that the density profile of the third layer at0.0720is much less peaked than the second layer at the same layer coverage,so any third layer solid phase must have very large zero-point motion.One possible consequence of the third layer entering solid-liquid coexistence is that it may fully solidify under compression of further adsorbed layers.As we have discussed in the previous section,the third layer density continues to increase even after fourth layer yer promotion is not a phase tran-sition and can occur whenever the chemical potential of the system favors it.Thus it is possible to have layer promotion in the middle of a phase transition.The third layer solid-liquid phase transition can be completed after fourth layer promotion as additional atoms added to the system get demoted to the third layer.We notefinally that there is some experimental evidence suggesting that third layer solidification may occur.In the heat capacity results presented by Greywall,8a small peak at about 1.8K can be observed for coverages beginning at0.3100 atom/˚A2,between the rounded heat capacity feature as-sociated with the fourth layer liquid and the sharp peak associated with the melting of the second layer solid. The fourth layer also exhibits a self-bound liquid cov-erage.We can identify this in the same manner as be-fore.Since promotion to the fourth layer occurs above 0.2837atom/˚A2,we can consider this coverage to cor-respond to a zero density fourth layer gas.We apply the Maxwell construction again to the fourth layer and determine a bound liquid phase at0.3345atom/˚A2(29 active atoms),giving a fourth layer equilibrium density of0.0508atom/˚A2.The solid line in thefigure gives the maximum possible range for gas-liquid coexistence in the fourth layer.All energy values in this region are on or above the coexistence line.This coexistence region agrees with heat capacity measurements,8which exhibit linear isotherms in the fourth layer up to0.3300atom/˚A2at low temperatures.A trend can be noticed in the equilibrium density as one progresses from the inner to outer layers.For thefirst layer of helium adsorbed on aflat substrate,we have cal-culated the equilibrium density to be0.0450(6)atom/˚A2, close to the two-dimensional value.30Successive increases are observed in the second and third layers,as the zero-point motion of the layers perpendicular to the plane of the substrate becomes larger.From a two-dimensional point of view,this motion has the effect of softening the hard cores of the helium atoms.In Fig.8,we do not plot energy values for the cov-erages between0.2413and0.2583atom/˚A2at400mK. Calculations that we performed for these densities at500 mK actually showed an energy decrease,outside of error bars.We attribute this to variations inherent in using frozen configurations for the second layer:the two calcu-lations required two different configurations,and so there will be some systematic difference in the energy values. We have repeated the third layer calculations at400 mK using a larger simulation cell(21.105˚A×20.889˚A). This will allow us to examinefinite-size effects and the effects of different configurations.We will also be able to examine the intermediate region that we excluded in the calculations using the smaller cell.The total energy values are given in Fig.9.These energy values have beenN E/N(K)0.22910.23130.24040.24720.25400.26760.27220.2767ρ0)2+C(ρ−ρ0e0(K)0.0097±0.51ρ0(˚A−2)0.2645±0.0005 B(K)2057.3±237.0 C(K)13885.7±1759.0χ2/ν 1.090.2500.2600.2700.2800.2900.3000.310density(A 2)0.500.600.700.800.901.00S u p e r f l u i d d e n s i t yFIG.12.Superfluid density values at 400mK.The frac-tion is relative to the number of active atoms.The arrow indicates the density at which fourth layer promotion begins.ond layer compression is not unexpected,since the com-pression of the first layer by the growth of the second is well established.32To allow for the effects of second layer compression,as well as the response of this layer to the growth of the third,we have performed calculations that include both second and third layer atoms in the Monte Carlo sampling.Density profiles illustrate the compression of the sec-ond layer in our simulation.These are shown in Fig.13for selected coverages.The left and right peaks in the fig-ure are for the second and third layers,respectively.As can be seen,the height of the second layer peak grows between the coverage 0.2296and 0.2879atom/˚A 2.The peak height then remains constant up to and including the coverage 0.2964,where promotion to the fourth layer is visible.Finally,the second layer peak increases again at the highest coverage examined,0.3049atom/˚A 2.By integrating the profiles up to the minimum be-tween the two peaks (at approximately 7.5˚A ),we ob-tain the second layer coverages of 0.0845,0.0885,0.0894,0.0890,and 0.0929atom/˚A 2for the coverages shown in the figure.Thus at the lowest third layer densities,no demotion occurs.Beginning at intermediate coverages and up to layer promotion,a single atom is demoted.At low fourth layer coverages (the highest coverage ex-amined)two atoms are demoted.As a consequence of demotion,promotion to the fourth layer is pushed to a higher overall coverage.As is illustrated in Fig.13,we do not observe promotion to the fourth layer until the den-sity has exceeded 0.2879atom/˚A 2,compared to 0.2837atom/˚A 2when the second layer was frozen.This value will be further established below by examining the den-sity dependence of the chemical potential.This value is still in agreement with the heat capacity 8and isother-mal compressibility 13,2measurements.We note that Zi-manyi,et al 15have proposed a slightly higher completion coverage of 0.293atom/˚A 2.3.08.013.0z(A)0.00.51.0ρ(z )FIG.13.Density profiles for the active second and third layers as a function of height above the substrate.The cov-erage values are 0.2296,0.2625,0.2879,0.2964,and 0.3045atom/˚A 2.The layer promotion illustrated above is accompanied by a discontinuity in the chemical potential.We can obtain this quantity by differencing our calculated total energy values,given in Table III B 1,and plot the results in Fig.14.Included in this figure are results obtained for the second layer from a previous calculation.4As can be seen,there is a distinctive change in the density depen-dence of the chemical potential µaround the promotion density.Below the promotion density,the energy changes very rapidly with increasing coverage.Near promotion,an added atom has the choice of going to the unoccupied third layer or the dense second layer,and will choose the layer that is energetically favorable.When the promotion coverage is reached,the chemical potential for adding the atom to the second layer exceeds that for adding it to the third layer,and so the atom is added to the unoccupied layer.It can be seen from the figure that the change in µassociated with the promotion is quite large.In this case,the second layer is solid and relatively dense,so we expect an energy gap to be associated with promotion.As can be seen in the density profiles (see Fig.13)for this layer,there is a range of heights above the second layer that is forbidden for the third layer atom.This is in contrast to what happens in the case of fourth layer pro-motion (see Fig.2),in which there is significantly more overlap between the third and fourth layers.Each addi-tional atom added to the system will also go the outer layer,but the rate of the energy change will decrease,since these atoms are attracted to each other and form a droplet.Thus we see that the chemical potential just after layer promotion decreases.。
