Sheng Meng

Easy-axis magnetic anisotropy separates two magnetic states with opposite magnetic moments, and single magnetic atoms and molecules with large easy-axis magnetic anisotropy are highly desired for future applications in high-density data storage and quantum computation. By tuning the metalation reaction between tetra-pyridyl-porphyrin molecules and Fe atoms, we have stabilized the so-called initial complex, an intermediate state of the reaction, on Au(111) substrate, and investigated the magnetic property of this complex at a single-molecule level by low-temperature scanning tunneling microscopy and spectroscopy. As revealed by inelastic electron tunneling spectroscopy in magnetic field, this Fe-porphyrin complex has magnetic anisotropy energy of more than 15 meV with its easy-axis perpendicular to the molecular plane. Two magnetic states with opposite spin directions are discriminated by the dependence of spin-flip excitation energy on magnetic field and are found to have long spin lifetimes. Our density functional theory calculations reveal that the Fe atom in this complex, decoupled from Au substrate by a weak ligand field with elongated Fe–N bonds, has a high-spin state S = 2 and a large orbital angular momentum L = 2, which give rise to easy-axis anisotropy perpendicular to the molecular plane and large magnetic anisotropy energy by spin-orbit coupling. Since the Fe atom is protected by the molecular ligand, the complex can be processed at room or even higher temperatures. The reported system may have potential applications in nonvolatile data storage, and our work demonstrates on-surface metalation reactions can be utilized to synthesize organometallic complexes with large magnetic anisotropy.Keywords: inelastic electron tunneling spectroscopy; magnetic anisotropy; on-surface reaction; organometallic complex; scanning tunneling microscopy; spin lifetime; spin-flip excitation;

Benign Interfacial Iodine Vacancies in Perovskite Solar Cells

Co-reporter:Lan-ying Wei, Wei Ma, Chao Lian, and Sheng Meng

The Journal of Physical Chemistry C March 23, 2017 Volume 121(Issue 11) pp:5905-5905

Publication Date(Web):February 27, 2017

DOI:10.1021/acs.jpcc.6b12583

Methylammonium lead iodide (MAPbI3) perovskite solar cells (PSCs) have become the forefront of photovoltaic technologies and attracted intense attention worldwide. MAPbI3 perovskites are mostly in the form of thin films in high-performance MAPbI3 PSCs, and charge transfer and other critical electronic dynamic processes take place at the interfaces of PSCs. The iodine vacancy VI is thought to play a major role in arousing severe hysteresis in photocurrent-photovoltage scan, which limits industrialization of PSCs. However, the surface and interfacial VI properties of MAPbI3 PSCs have not been systematically studied. We utilize first-principles method and nonadiabatic electron dynamics simulations to study the structural and electronic properties of VI at various sites of freestanding MAPbI3 film and the MAPbI3/TiO2 heterojunction. We show that the surface and interfacial VI are more stable than bulk, in agreement with accumulation of VI at grain boundaries observed in experiments. The migration of VI in the perovskite layer under electric field during voltage scans contributes to the anomalous hysteresis in PSCs. VI at Pb–I layer and MA–I layer are quite different: VI at MA–I layer are more stable, while VI defect states at Pb–I layer are more local and weakly covalent bonded. VI promotes both electronic injection and recombination rates, but overall reduces the power conversion efficiencies (PCE) of PSCs. Nevertheless, interfacial VI is found to be the least harmful to the PCE of PSCs comparing with random sites in the bulk, contributing to the high PCE of MAPbI3 PSCs.

Monitoring Local Strain Vector in Atomic-Layered MoSe2 by Second-Harmonic Generation

Co-reporter:Jing Liang, Jin Zhang, Zhenzhu Li, Hao Hong, Jinhuan Wang, Zhihong Zhang, Xu Zhou, Ruixi Qiao, Jiyu Xu, Peng Gao, Zhirong Liu, Zhongfan Liu, Zhipei Sun, Sheng Meng, Kaihui Liu, and Dapeng Yu

Nano Letters December 13, 2017 Volume 17(Issue 12) pp:7539-7539

Publication Date(Web):November 22, 2017

DOI:10.1021/acs.nanolett.7b03476

Strain serves as a powerful freedom to effectively, reversibly, and continuously engineer the physical and chemical properties of two-dimensional (2D) materials, such as bandgap, phase diagram, and reaction activity. Although there is a high demand for full characterization of the strain vector at local points, it is still very challenging to measure the local strain amplitude and its direction. Here, we report a novel approach to monitor the local strain vector in 2D molybdenum diselenide (MoSe2) by polarization-dependent optical second-harmonic generation (SHG). The strain amplitude can be evaluated from the SHG intensity in a sensitive way (−49% relative change per 1% strain); while the strain direction can be directly indicated by the evolution of polarization-dependent SHG pattern. In addition, we employ this technique to investigate the interlayer locking effect in 2H MoSe2 bilayers when the bottom layer is under stretching but the top layer is free. Our observation, combined with ab initio calculations, demonstrates that the noncovalent interlayer interaction in 2H MoSe2 bilayers is strong enough to transfer the strain of at least 1.4% between the bottom and top layers to prevent interlayer sliding. Our results establish that SHG is an effective approach for in situ, sensitive, and noninvasive measurement of local strain vector in noncentrosymmetric 2D materials.Keywords: 2D materials; MoSe2; second-harmonic generation; strain;

High-Efficiency Photovoltaic Conversion at Selective Electron Tunneling Heterointerfaces

Co-reporter:Chuancheng Jia;Wei Ma;Jianxin Guan;Chunhui Gu;Xinxi Li;Linan Meng;Yao Gong;Xuefeng Guo

Advanced Electronic Materials 2017 Volume 3(Issue 11) pp:

Publication Date(Web):2017/11/01

DOI:10.1002/aelm.201700211

AbstractEffectively controlling photoinduced charge transport at the heterointerface is of crucial importance for improving the performance of photovoltaic devices. On the basis of an ipsilateral selective electron tunneling (ISET) mechanism, here this study investigates photoinduced charge transport and photovoltaic conversion at a simplified dye/single-layer graphene (SLG)/TiO2 ternary interface. With an amphiphilic Z907 molecule as the model dye, the photoexcited electrons in the dye can directly tunnel across SLG and be collected by the TiO2 layer with an efficiency of 96.23%, which guarantees a high-efficiency photoelectric conversion at the ISET-based heterointerface. More importantly, the intrinsic Schottky barrier and fast hole collection rate at the heterointerface lead to a high photovoltage, a large fill factor, and the good intense-light performance for photovoltaic conversion. Such an ISET-based heterointerface may offer a platform of designing and developing a novel class of photovoltaic devices with high efficiency.

Exotic thermoelectric behavior in nitrogenated holey graphene

Co-reporter:Yinchang Zhao;Zhenhong Dai;Chao Lian,

RSC Advances (2011-Present) 2017 vol. 7(Issue 42) pp:25803-25810

Publication Date(Web):2017/05/15

DOI:10.1039/C7RA03597G

Combining first-principles approaches with the Boltzmann transport equation and semi-classical analysis we investigate the thermal conductivity κ, thermopower S, electrical conductivity σ, and carrier mobility μ of newly synthesized nitrogenated holey graphene (NHG). Strikingly, the NHG possesses exceedingly high S and σ but a fairly low lattice thermal conductivity κL, and therefore extraordinary thermoelectric properties, with a figure of merit zT even exceeding 5.0, are obtained in the n-type doped NHG. The outstanding thermoelectric behavior of NHG is attributed to its exotic atomic and electronic structure: (i) strong anharmonic phonon scattering results in a very low κL; (ii) flat bands around the Fermi level together with a large band gap cause high S; and (iii) conduction band dipping at the Brillouin zone center leads to a high electron mobility μ and thus high σ.

A modified Wenzel model for water wetting on van der Waals layered materials with topographic surfaces

Co-reporter:Peng Zhao;Yongfeng Huang;Yutian Shen;Shuo Yang;Lan Chen;Kehui Wu;Hui Li

Nanoscale (2009-Present) 2017 vol. 9(Issue 11) pp:3843-3849

Publication Date(Web):2017/03/17

DOI:10.1039/C7NR00521K

A modified Wenzel model is proposed for describing the wetting behavior of van der Waals layered materials with topographic surfaces, based on the measured linear relationship between water wetting and surface roughness for high quality Bi2Se3 thin films, synthesized using molecular beam epitaxy (MBE) in the optimized temperature window of 180–200 °C. The water contact angles are found to have apparent dependence on the nanoscale surface morphology, enabling film wettability as a new tool to quickly characterize the quality of atomically thin films. The water contact angle of the ideal Bi2Se3 surface is inferred to be ∼98.4°, indicating its intrinsic hydrophobic nature; however, the edge of the terrace on its surface is extremely hydrophilic, leading to easy hydrophobic/hydrophilic transitions. The atomistic mechanism is further revealed by first principles calculations. The regulated wettability is of great importance for electronic applications of Bi2Se3 and other two-dimensional materials with distinctive electronic structures.

High-Efficiency Selective Electron Tunnelling in a Heterostructure Photovoltaic Diode

Co-reporter:Chuancheng Jia, Wei Ma, Chunhui Gu, Hongliang Chen, Haomiao Yu, Xinxi Li, Fan Zhang, Lin Gu, Andong Xia, Xiaoyuan Hou, Sheng Meng, and Xuefeng Guo

Nano Letters 2016 Volume 16(Issue 6) pp:3600-3606

Publication Date(Web):May 16, 2016

DOI:10.1021/acs.nanolett.6b00727

A heterostructure photovoltaic diode featuring an all-solid-state TiO2/graphene/dye ternary interface with high-efficiency photogenerated charge separation/transport is described here. Light absorption is accomplished by dye molecules deposited on the outside surface of graphene as photoreceptors to produce photoexcited electron–hole pairs. Unlike conventional photovoltaic conversion, in this heterostructure both photoexcited electrons and holes tunnel along the same direction into graphene, but only electrons display efficient ballistic transport toward the TiO2 transport layer, thus leading to effective photon-to-electricity conversion. On the basis of this ipsilateral selective electron tunnelling (ISET) mechanism, a model monolayer photovoltaic device (PVD) possessing a TiO2/graphene/acridine orange ternary interface showed ∼86.8% interfacial separation/collection efficiency, which guaranteed an ultrahigh absorbed photon-to-current efficiency (APCE, ∼80%). Such an ISET-based PVD may become a fundamental device architecture for photovoltaic solar cells, photoelectric detectors, and other novel optoelectronic applications with obvious advantages, such as high efficiency, easy fabrication, scalability, and universal availability of cost-effective materials.

Interfacial Oxygen Vacancies as a Potential Cause of Hysteresis in Perovskite Solar Cells

Co-reporter:Fan Zhang, Wei Ma, Haizhong Guo, Yicheng Zhao, Xinyan Shan, Kuijuan Jin, He Tian, Qing Zhao, Dapeng Yu, Xinghua Lu, Gang Lu, and Sheng Meng

Chemistry of Materials 2016 Volume 28(Issue 3) pp:802

Publication Date(Web):January 13, 2016

DOI:10.1021/acs.chemmater.5b04019

Organometal halide perovskite solar cells (PSCs) have emerged as one of the most promising photovoltaic technologies with efficiencies exceeding 20.3%. However, device stability problems including hysteresis in current–voltage scans must be resolved before the commercialization of PSCs. Transient absorption measurements and first-principles calculations indicate that the migration of oxygen vacancies in the TiO2 electrode under electric field during voltage scans contributes to the anomalous hysteresis in PSCs. The accumulation of oxygen vacancies at the electrode/perovskite interface slows down charge extraction while significantly speeding up charge recombination at the interface. Moreover, nonadiabatic molecular dynamics simulations reveal that the charge recombination rates at the interface depend sensitively (with 1 order of magnitude difference) on the locations of oxygen vacancies. By intentionally reducing oxygen vacancies in the TiO2 electrode, we substantially suppress unfavorable hysteresis in the PSC devices. This work establishes a firm link between microscopic interfacial structure and macroscopic device performance of PSCs, providing important clues for future device design and optimization.

Tunable magnetic moment and potential half-metal behavior of Fe-nanostructure-embedded graphene perforation

Co-reporter:Huizhen Zhang, Jia-Tao Sun, Haifang Yang, Lin Li, Huixia Fu, Sheng Meng, Changzhi Gu

Carbon 2016 Volume 107() pp:268-272

Publication Date(Web):October 2016

DOI:10.1016/j.carbon.2016.06.001

Doping with transition metal elements is an effective method to introduce magnetism in graphene, which could enable future graphene-based spintronic devices. Motivated by the recent experimental observation of a stable single layer iron membrane embedded in graphene perforation, we investigate the electronic and magnetic properties of the Fe-nanostructure-embedded graphene system based on first principles calculations. The results demonstrate that strain could lead to dramatic changes in the magnetic configurations for both small Fe clusters bonded to the edge carbon atoms of graphene perforation and the single layer Fe membrane fully embedded in the graphene layer. For optimal doping, a delicate balance can be achieved, which leads to a half-metallic electronic structure. This work suggests an easy and effective method to introduce and tune the magnetic properties of graphene, which offers a new direction for the development of future graphene-based spintronic devices.

Long-Lived Multifunctional Superhydrophobic Heterostructure Via Molecular Self-Supply

Co-reporter:Yongfeng Huang;Ying Hu;Chongqin Zhu;Fan Zhang;Hui Li;Xinghua Lu

Advanced Materials Interfaces 2016 Volume 3( Issue 5) pp:

Publication Date(Web):

DOI:10.1002/admi.201500727

Superhydrophobic (SHO) surfaces have drawn great attention thanks to their theoretical significance and myriad applications in industry and everyday life. Current approaches to fabricate such surfaces require calcinating at high temperatures, tedious and time-consuming treatments, toxic chemicals, and/or processing with intricate instruments. Long-duration SHO surfaces are even more challenging due to material instability and easy contamination by organic pollutants in dry conditions. To overcome these difficulties we design a simple approach via self-supplying of low surface tension chemicals to nanoparticles to fabricate multifunctional SHO heterostructures. The method herein features room temperature, rapid processing, with environment-friendly raw materials. With multiple functions such as photocatalysis and transparency SHO surfaces further extend their lifetime and enable self-sustaining environment maintenance.

Recent progresses in real-time local-basis implementation of time dependent density functional theory for electron–nucleus dynamics

Co-reporter:Wei Ma, Jin Zhang, Lei Yan, Yang Jiao, Yi Gao, Sheng Meng

Computational Materials Science 2016 Volume 112(Part B) pp:478-486

Publication Date(Web):1 February 2016

DOI:10.1016/j.commatsci.2015.08.056

•An efficient time-dependent density functional theory simulation method is presented.•Electron–nucleus dynamics beyond Born–Oppenheimer approximation can be simulated.•Real time propagation and local basis sets are employed.•The method works well for absorption, charge injection, excited state bond breaking.We present an efficient real-time time-dependent density functional theory (TDDFT) method for large-scale accurate simulations of electron–nucleus dynamics, as implemented in the time dependent ab-initio package (TDAP). By employing a local basis-set presentation, we are able to simulate systems of large size (∼500 atoms) and for long electronic propagation time (∼300–500 fs) with less computation cost while maintaining relatively high accuracy. We show several quintessential examples, such as photoabsorption spectra of dye-sensitized TiO2 nanowire, proton transfer coupled nonradiative relaxation of eumelanin constituents, electron injection and electron–hole recombination in dye solar cells, hole-transfer dynamics between MoS2/WS2 interlayer heterojunction, and solvent effects on electron dynamics. Our method is demonstrated to have superiority over available methods in dealing with interesting excited state characteristics of complex systems involving dynamics of electrons and atoms.

Novel Excitonic Solar Cells in Phosphorene–TiO2 Heterostructures with Extraordinary Charge Separation Efficiency

Co-reporter:Liujiang Zhou; Jin Zhang; Zhiwen Zhuo; Liangzhi Kou; Wei Ma; Bin Shao; Aijun Du; Sheng Meng;Thomas Frauenheim

The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 10) pp:1880-1887

Publication Date(Web):May 4, 2016

DOI:10.1021/acs.jpclett.6b00475

Constructing van der Waals heterostructures is an efficient approach to modulate the electronic structure, to advance the charge separation efficiency, and thus to optimize the optoelectronic property. Here, we theoretically investigated the phosphorene interfaced with TiO2(110) surface (1L-BP/TiO2) with a type-II band alignment, showing enhanced photoactivity. The 1L-BP/TiO2 excitonic solar cell (XSC) based on the 1L-BP/TiO2 exhibits large built-in potential and high power conversion efficiency (PCE), dozens of times higher than conventional solar cells, comparable to MoS2/WS2 XSC. The nonadiabatic molecular dynamics simulation shows the ultrafast electron transfer time of 6.1 fs, and slow electron–hole recombination of 0.58 ps, yielding >98% internal quantum efficiency for charge separation, further guaranteeing the practical PCE. Moreover, doping in phosphorene has a tunability on built-in potential, charge transfer, light absorbance, as well as electron dynamics, which greatly helps to optimize the optoelectronic efficiency of a XSC.

Inducing Transient Charge State of a Single Water Cluster on Cu(111) Surface

Co-reporter:Yang Guo, Zijing Ding, Lihuan Sun, Jianmei Li, Sheng Meng, and Xinghua Lu

ACS Nano 2016 Volume 10(Issue 4) pp:4489

Publication Date(Web):March 23, 2016

DOI:10.1021/acsnano.6b00230

The hydrated electron on solid surface is a crucial species to interfacial chemistry. We present a joint low-temperature scanning tunneling microscopy and density functional theory investigation to explore the existence of a transient hydrated electron state induced by injecting tunneling electrons into a single water nonamer cluster on Cu(111) surface. The directional diffusion of water cluster under the Coulomb repulsive potential has been observed as evidence for the emergence of the transient hydrated electron. A critical structure transformation in water cluster for the emergence of hydrated electron has been identified. A charging mechanism has been proposed based on density functional theory calculation and scanning tunneling microscope results.Keywords: charge state; Cu(111) surface; density functional theory; hydrated electron; scanning tunneling microscopy; water cluster

Quantum Mode Selectivity of Plasmon-Induced Water Splitting on Gold Nanoparticles

Co-reporter:Lei Yan, Fangwei Wang, and Sheng Meng

ACS Nano 2016 Volume 10(Issue 5) pp:5452

Publication Date(Web):April 29, 2016

DOI:10.1021/acsnano.6b01840

Plasmon induced water splitting is a promising research area with the potential for efficient conversion of solar to chemical energy, yet its atomic mechanism is not well understood. Here, ultrafast electron–nuclear dynamics of water splitting on gold nanoparticles upon exposure to femtosecond laser pulses was directly simulated using real time time-dependent density functional theory (TDDFT). Strong correlation between laser intensity, hot electron transfer, and reaction rates has been identified. The rate of water splitting is dependent not only on respective optical absorption strength, but also on the quantum oscillation mode of plasmonic excitation. Odd modes are more efficient than even modes, owing to faster decaying into hot electrons whose energy matches well the antibonding orbital of water. This finding suggests photocatalytic activity can be manipulated by adjusting the energy level of plasmon-induced hot carriers, through altering the cluster size and laser parameter, to better overlap adsorbate unoccupied level in plasmon-assisted photochemistry.Keywords: hot electron; photosplitting; quantum plasmon mode; time-dependent density functional theory; ultrafast dynamics

Multilayered silicene: the bottom-up approach for a weakly relaxed Si(111) with Dirac surface states

Co-reporter:Huixia Fu, Lan Chen, Jian Chen, Jinglan Qiu, Zijing Ding, Jin Zhang, Kehui Wu, Hui Li and Sheng Meng

Nanoscale 2015 vol. 7(Issue 38) pp:15880-15885

Publication Date(Web):10 Aug 2015

DOI:10.1039/C5NR04548G

Combining first principles investigations and scanning tunneling microscopy, we identify that the presumable van der Waals packed multilayered silicene sheets spontaneously transform into a diamond-structure bulk Si film due to strong interlayer couplings. In contrast to drastic surface reconstruction on conventional Si(111), multilayered silicene prepared by bottom-up epitaxy on Ag(111) exhibits a nearly ideal flat surface with only weak buckling. Without invoking Ag surfactants, √3 × √3 honeycomb patterns emerge thanks to dynamic fluctuation of mirror-symmetric rhombic phases, similar to monolayered silicene [Chen et al., Phys. Rev. Lett., 2013, 110, 085504]. The weak relaxation enables novel surface states with a Dirac linear dispersion.

“H2 sponge”: pressure as a means for reversible high-capacity hydrogen storage in nanoporous Ca-intercalated covalent organic frameworks

Co-reporter:Fei Gao, Jia Tao Sun and Sheng Meng

Nanoscale 2015 vol. 7(Issue 14) pp:6319-6324

Publication Date(Web):06 Mar 2015

DOI:10.1039/C4NR07447E

We explore the potential and advantages of Ca-intercalated covalent organic framework-1 (CaCOF-1) as a 3-dimensional (3D) layered material for reversible hydrogen storage. Density functional theory calculations show that by varying the interlayer distance of CaCOF-1, a series of metastable structures can be achieved with the interlayer distance falling in the range of 4.3–4.8 Å. When four hydrogen molecules are adsorbed on each Ca, a high hydrogen uptake of 4.54 wt% can be produced, with the binding energy falling in the ideal range of 0.2–0.6 eV per H2. While H2 absorption is a spontaneous process under H2 rich conditions, tuning the interlayer distance by reasonable external pressure could compress CaCOF-1 to release all of the hydrogen molecules and restore the material to its original state for recyclable use. This provides a new method for gradual, controllable extraction of hydrogen molecules in covalent organic frameworks, satisfying the practical demand for reversible hydrogen storage at ambient temperatures.

Basic science of water: Challenges and current status towards a molecular picture

Co-reporter:Sheng Meng;Lauren F. Greenlee;Yuen Ron Shen;Enge Wang

Nano Research 2015 Volume 8( Issue 10) pp:3085-3110

Publication Date(Web):2015 October

DOI:10.1007/s12274-015-0822-y

Rapid developments in both fundamental science and modern technology that target water-related problems, including the physical nature of our planet and environment, the origin of life, energy production via water splitting, and water purification, all call for a molecular-level understanding of water. This invokes relentless efforts to further our understanding of the basic science of water. Current challenges to achieve a molecular picture of the peculiar properties and behavior of water are discussed herein, with a particular focus on the structure and dynamics of bulk and surface water, the molecular mechanisms of water wetting and splitting, application-oriented research on water decontamination and desalination, and the development of complementary techniques for probing water at the nanoscale.

Precise Identification and Manipulation of Adsorption Geometry of Donor−π–Acceptor Dye on Nanocrystalline TiO2 Films for Improved Photovoltaics

Co-reporter:Fan Zhang, Wei Ma, Yang Jiao, Jingchuan Wang, Xinyan Shan, Hui Li, Xinghua Lu, and Sheng Meng

ACS Applied Materials & Interfaces 2014 Volume 6(Issue 24) pp:22359

Publication Date(Web):November 24, 2014

DOI:10.1021/am506365a

Adsorption geometry of dye molecules on nanocrystalline TiO2 plays a central role in dye-sensitized solar cells, enabling effective sunlight absorption, fast electron injection, optimized interface band offsets, and stable photovoltaic performance. However, precise determination of dye binding geometry and proportion has been challenging due to complexity and sensitivity at interfaces. Here employing combined vibrational spectrometry and density functional calculations, we identify typical adsorption configurations of widely adopted cyanoacrylic donor-π bridge-acceptor dyes on nanocrystalline TiO2. Binding mode switching from bidentate bridging to hydrogen-bonded monodentate configuration with Ti–N bonding has been observed when dye-sensitizing solution becomes more basic. Raman and infrared spectroscopy measurements confirm this configuration switch and determine quantitatively the proportion of competing binding geometries, with vibration peaks assigned using density functional theory calculations. We further found that the proportion of dye-binding configurations can be manipulated by adjusting pH value of dye-sensitizing solutions. Controlling molecular adsorption density and configurations led to enhanced energy conversion efficiency from 2.4% to 6.1% for the fabricated dye-sensitized solar cells, providing a simple method to improve photovoltaic performance by suppressing unfavorable binding configurations in solar cell applications.Keywords: adsorption structure; cyanoacrylic acid; dye-sensitized solar cell; enhanced efficiency; manipulation; vibrational spectra

Predicting Energy Conversion Efficiency of Dye Solar Cells from First Principles

Co-reporter:Wei Ma ; Yang Jiao

The Journal of Physical Chemistry C 2014 Volume 118(Issue 30) pp:16447-16457

Publication Date(Web):January 9, 2014

DOI:10.1021/jp410982e

In this work we target on accurately predicting energy conversion efficiency of dye-sensitized solar cells (DSC) using parameter-free first principles simulations. We present a set of algorithms, mostly based on solo first principles calculations within the framework of density functional theory, to accurately calculate key properties in energy conversion including sunlight absorption, electron injection, electron–hole recombination, open circuit voltages, and so on. We choose two series of donor-π-acceptor dyes with detailed experimental photovoltaic data as prototype examples to show how these algorithms work. Key parameters experimentally measured for DSC devices can be nicely reproduced by first-principles with as less empirical inputs as possible. For instance, short circuit current of model dyes can be well reproduced by precisely calculating their absorption spectra and charge separation/recombination rates. Open circuit voltages are evaluated through interface band offsets, namely, the difference between the Fermi level of electrons in TiO2 and the redox potential of the electrolyte, after modification with empirical formulas. In these procedures the critical photoelectron injection and recombination dynamics are calculated by real-time excited state electronic dynamics simulations. Estimated solar cell efficiency reproduces corresponding experimental values, with errors usually below 1–2%. Device characteristics such as light harvesting efficiency, incident photon-to-electron conversion efficiency, and the current–voltage characteristics can also be well reproduced and compared with experiment. Thus, we develop a systematic ab initio approach to predict solar cell efficiency and photovoltaic performance of DSC, which enables large-scale efficient dye screening and optimization through high-throughput first principles calculations with only a few parameters taken from experimental settings for electrode and electrolyte toward a renewable energy based society.

Correction to Adsorption and Diffusion of Lithium on Layered Silicon for Li-Ion Storage

Co-reporter:Georgios A. Tritsaris, Efthimios Kaxiras, Sheng Meng, and Enge Wang

Nano Letters 2013 Volume 13(Issue 6) pp:3004-3004

Publication Date(Web):May 24, 2013

DOI:10.1021/nl4018644

StructureProperty Relations in All-Organic Dye-Sensitized Solar Cells

Co-reporter:Yang Jiao;Fan Zhang;Michael Grätzel

Advanced Functional Materials 2013 Volume 23( Issue 4) pp:424-429

Publication Date(Web):

DOI:10.1002/adfm.201201831

Abstract

A structure–property relationship in all-organic dye solar cells is revealed by first-principles molecular dynamics and real-time time-dependent density functional theory simulations, accompanied with experimental confirmation. An important structural feature at the interface, Ti–N anchoring, for a broad group of all-organic dyes on TiO₂ is inferred from energetics, vibrational recognition, and electronic data. This fact is contrary to the usual assumption; however, it optimizes electronic level alignment and photoelectron injection dynamics, greatly contributing to the observed efficiency improvement in all-organic cyanoacrylate dye sensitized solar cells.

Modeling charge recombination in dye-sensitized solar cells using first-principles electron dynamics: effects of structural modification

Co-reporter:Wei Ma, Yang Jiao and Sheng Meng

Physical Chemistry Chemical Physics 2013 vol. 15(Issue 40) pp:17187-17194

Publication Date(Web):16 Aug 2013

DOI:10.1039/C3CP52458B

We have performed real-time excited state simulations of electron injection and charge recombination at a dye/semiconductor interface within the framework of time-dependent density functional theory (TDDFT). We found that by inserting a phenyl ring into the organic dye, the charge recombination rate is slowed down by about four times, while the injection rate keeps almost the same. This introduces a drastic increase in the energy conversion efficiency by several folds, in agreement with experimental observations. Quantum simulations thus provide a new way to understand the role of the dye's building blocks and offer new strategies to optimize individual energy transfer steps for improving the efficiency in renewable energy applications.

Quinoid conjugated dye designed for efficient sensitizer in dye sensitized solar cells

Co-reporter:Yang Jiao, Wei Ma, Sheng Meng

Chemical Physics Letters 2013 Volume 586() pp:97-99

Publication Date(Web):24 October 2013

DOI:10.1016/j.cplett.2013.09.008

•Quinoid conjugated dye was introduced as sensitizer in dye sensitized solar cells.•Near infrared light harvesting and high energy conversion efficiency was theoretically predicted.•Interface electron hole separation dynamics were simulated using time-dependent density functional theory.Paraquinoid rings are introduced in the π-conjugation of all-organic donor-π-acceptor dyes as sensitizer in dye sensitized solar cells, to drastically shift optical response from violet-blue to near-infrared and to significantly enhance photoabsorption. Taking Y1 as a model, real time electron dynamics simulations based on time-dependent density functional theory confirm that paraquinoid conjugation maintains high thermal stability and ultrafast electron-hole separation at ambient temperature.

Controlling Adsorption Structure of Eosin Y Dye on Nanocrystalline TiO2 Films for Improved Photovoltaic Performances

Co-reporter:Fan Zhang, Feng Shi, Wei Ma, Fei Gao, Yang Jiao, Hui Li, Jingchuan Wang, Xinyan Shan, Xinghua Lu, and Sheng Meng

The Journal of Physical Chemistry C 2013 Volume 117(Issue 28) pp:14659-14666

Publication Date(Web):June 21, 2013

DOI:10.1021/jp404439p

Adsorption structure of Eosin Y dyes on nanocrystalline TiO2 can be manipulated by adding a small fraction of water into organic electrolyte. Binding mode switching from hydrogen bonded monodentate to bidentate bridging configuration has been observed and confirmed by Raman and infrared spectroscopy measurements, with vibration peaks assigned using density functional theory calculations. Photovoltaic measurements on the fabricated dye-sensitized solar cells indicate that energy conversion efficiency is enhanced by manipulating molecular adsorption configuration of Eosin Y dyes. This opens a new avenue to improving photovoltaic performance by suppressing unfavorable adsorption configurations in dye solar cell devices.

First Principles Design of Dye Molecules with Ullazine Donor for Dye Sensitized Solar Cells

Co-reporter:Jie Feng, Yang Jiao, Wei Ma, Md. K. Nazeeruddin, Michael Grätzel, and Sheng Meng

The Journal of Physical Chemistry C 2013 117(8) pp: 3772-3778

Publication Date(Web):February 7, 2013

DOI:10.1021/jp310504n

We design a series of metal-free donor-π-bridge molecules (denoted VB0–VB4) based on a new donor group—ullazine donor—as sensitizers for dye sensitized solar cell (DSSC) applications. Density functional theory (DFT) and time-dependent DFT calculations reveal that the physical properties of dyes, including spectral response, light harvesting efficiency, and electron injection rate, are systematically improved by combining ullazine donor to a series of length changing π bridges. Dye VB2 is the best candidate thanks to its outstanding performance on key parameters and achieving a balance between competing factors. Compared to two other series of molecules—L and M dyes, which differ from VB dyes by only the donor group—VB dyes have the largest light harvesting efficiency and the largest number of electrons injected to the conduction band of TiO2. These results suggest that the ullazine group can serve as an excellent donor for future DSSC applications.

Bis(pyrazol-1-yl)methane as Non-Chromophoric Ancillary Ligand for Charged Bis-Cyclometalated Iridium(III) Complexes

Co-reporter:Sheng Meng;Il Jung;Jie Feng;Rosario Scopelliti;Davide Di Censo;Michael Grätzel;M. Khaja Nazeeruddin;Etienne Baranoff

European Journal of Inorganic Chemistry 2012 Volume 2012( Issue 19) pp:3209-3215

Publication Date(Web):

DOI:10.1002/ejic.201200197

Abstract

New charged cyclometalated iridium(III) complexes [Ir(ppy)₂(L)](PF₆) [ppy = 2-phenylpyridine; L = bis(pyrazol-1-yl)methane (for 1); L = bis(3,5-dimethylpyrazol-1-yl)methane (for 2)] were synthesized and their electrochemical and photophysical properties studied. These complexes with non-π-electron-conjugated ancillary chelates exhibit significantly blueshifted emission relative to those of commonly used derivatives with NN ancillary ligands such as bipyridine or phenanthroline. Both X-ray and theoretical analysis based on time-dependent density functional theory (TD-DFT) reveal that the binding of Ir to the bis(pyrazol-1-yl)methane ancillary ligand is much weaker than that to the phenylpyridine main ligand; the effect is enhanced in the excited state. As a result, the ancillary ligand does not participate in low-energy excitations and triplet emission, and the electronic transitions are concentrated on the main chromophoric ligands. The blueshift feature is attributed to emission originating from the main cyclometalated ligands, in contrast to emitters with the NN chromophoric ancillary ligand. In addition, complex 2 exhibits a one order of magnitude higher non-radiative decay rate than complex 1, which is attributed to the steric hindrance of the methyl groups that leads to a more loosely bound ancillary ligand.

Water wetting on representative metal surfaces: Improved description from van der Waals density functionals

Co-reporter:Ikutaro Hamada, Sheng Meng

Chemical Physics Letters 2012 Volume 521() pp:161-166

Publication Date(Web):10 January 2012

DOI:10.1016/j.cplett.2011.11.070

Abstract

We present a simple and useful approach based on the van der Waals density functional to investigate water wetting on two representative metal surfaces, Cu(1 1 0) and Ru(0 0 0 1). We found that a mixed van der Waals density functional, by incorporating the Perdew–Burke–Ernzerhof (PBE) exchange for water–metal interactions and the revised PBE (revPBE) exchange of Zhang and Yang for hydrogen bonding, respectively, correctly predicts the wetting of extended water-chains and half-dissociated water layer on Ru(0 0 0 1), as well as wetting of H-down bilayers on Cu(1 1 0), after correcting zero-point energy.

Theoretical investigation of the C60/copper phthalocyanine organic photovoltaic heterojunction

Co-reporter:Jun Ren;Efthimios Kaxiras

Nano Research 2012 Volume 5( Issue 4) pp:248-257

Publication Date(Web):2012 April

DOI:10.1007/s12274-012-0204-7

Ultrasmall Silver Nanopores Fabricated by Femtosecond Laser Pulses

Co-reporter:F. Bian, Y. C. Tian, R. Wang, H. X. Yang, Hongxing Xu, Sheng Meng, and Jimin Zhao

Nano Letters 2011 Volume 11(Issue 8) pp:3251-3257

Publication Date(Web):June 30, 2011

DOI:10.1021/nl201529d

Ultrasmall nanopores in silver thin films with a diameter of about 2 nm have been fabricated using femtosecond laser ablation in liquid. Ultrafast laser pulse ablation generates highly nonequilibrium excitated states, from which silver thin films emerge and progressively grow with the assistance of capping agent molecules. During this growth process, capping agent molecules are enclaved within the film, leaving individual ultrasmall pores in the thin film. Our first-principles calculations show that the pore size is critically determined by the dimension of the confined molecules. Our approach advances the capability of optical methods in making nanoscale structures with potential applications in areas such as near-field aperture probes, imaging masks, magnetic plasmonic resonances, and biosensing with individual nanopores.

Atomistic mechanism of charge separation upon photoexcitation at the dye–semiconductor interface for photovoltaic applications

Co-reporter:Yang Jiao, Zijing Ding and Sheng Meng

Physical Chemistry Chemical Physics 2011 vol. 13(Issue 29) pp:13196-13201

Publication Date(Web):28 Jun 2011

DOI:10.1039/C1CP20540D

Charge separation in excited states upon visible light absorption is a central process in photovoltaic solar cell applications. Employing state-of-the-art first principles calculations based on time-dependent density functional theory (TDDFT), we simulate electron–hole dynamics in real time and illustrate the microscopic mechanism of charge separation at the interface between organic dye molecules and oxide semiconductor surfaces in dye-sensitized solar cells. We found that electron–hole separation proceeds non-adiabatically on an ultrafast timescale <100 fs at an anthocyanin/TiO2 interface, and it is strongly mediated by the vibrations of interface Ti–O bonds, which anchor the dye onto the TiO2 surface. The obtained absorption spectrum and electron injection timescale agree with experimental measurements.

Design of Dye Acceptors for Photovoltaics from First-Principles Calculations

Co-reporter:Sheng Meng ; Efthimios Kaxiras ; Md. K. Nazeeruddin ;Michael Grätzel

The Journal of Physical Chemistry C 2011 Volume 115(Issue 18) pp:9276-9282

Publication Date(Web):April 18, 2011

DOI:10.1021/jp201646q

We investigate a set of donor-π-acceptor (D-π-A) dyes with new acceptor groups for dye-sensitized solar cells, using time-dependent density-functional-theory calculations of the electronic structure and optical absorption. We considered three types of modifications on existing dye structures: (i) replacement of the side cyano group (CN) on the molecular anchor, (ii) insertion and alteration of the intermediate spacer groups, and (iii) modification of the number and positions of cyano CN groups on a phenyl-ring spacer. We find that with these modifications, the dye electronic levels and corresponding optical absorption properties can be gradually tuned, rendering possible the identification of dyes with desirable structural, electronic, and optical properties. For example, dyes with phenyl and CN-substituted phenyl groups are promising candidates for red light absorption and high molar extinction coefficients.

Electron and Hole Dynamics in Dye-Sensitized Solar Cells: Influencing Factors and Systematic Trends

Co-reporter:Sheng Meng and Efthimios Kaxiras

Nano Letters 2010 Volume 10(Issue 4) pp:1238-1247

Publication Date(Web):March 30, 2010

DOI:10.1021/nl100442e

We investigate electron and hole dynamics upon photon excitation in dye-sensitized solar cells, using a recently developed method based on real-time evolution of electronic states through time-dependent density functional theory. The systems we considered consist of organic sensitizers and nanocrystalline TiO2 semiconductors. We examine the influence of various factors on the dynamics of electrons and holes, including point defects (vacancies) on the TiO2 surface, variations in the dye molecular size and binding geometry, and thermal fluctuations which result in different alignments of the electronic energy levels. Two clear trends emerge: (a) dissociated adsorption of the dye molecules leads to faster electron injection dynamics by reducing interfacial dipole moments; (b) oxygen vacancy defects stabilize dye adsorption and facilitate charge injection, at the cost of lower open circuit voltage and higher electron−hole recombination rate. Understanding of these effects at the atomic level suggests tunable parameters through which the electronic characteristics of dye-sensitized solar cell devices can be improved and their efficiency can be maximized.

Atomistic mechanism of charge separation upon photoexcitation at the dye–semiconductor interface for photovoltaic applications

Co-reporter:Yang Jiao, Zijing Ding and Sheng Meng

Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 29) pp:NaN13201-13201

Publication Date(Web):2011/06/28

DOI:10.1039/C1CP20540D

Modeling charge recombination in dye-sensitized solar cells using first-principles electron dynamics: effects of structural modification

Co-reporter:Wei Ma, Yang Jiao and Sheng Meng

Physical Chemistry Chemical Physics 2013 - vol. 15(Issue 40) pp:NaN17194-17194

Publication Date(Web):2013/08/16

DOI:10.1039/C3CP52458B