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2020 (Advanced Materials)

Gate‐Tunable Polar Optical Phonon to Piezoelectric Scattering in Few‐Layer Bi2O2Se for High‐Performance Thermoelectrics

Abstract:Atomically thin Bi2O2Se has emerged as a new member in 2D materials with ultrahigh carrier mobility and excellent air‐stability, showing great potential for electronics and optoelectronics. In addition, its ferroelectric nature renders an ultralow thermal conductivity, making it a perfect candidate for thermoelectrics. In this work, the thermoelectric performance of 2D Bi2O2Se is investigated over a wide temperature range (20–300 K). A gate‐tunable transition from polar optical phonon (POP) scattering to piezoelectric scattering is observed, which facilitates the capacity of drastic mobility engineering in 2D Bi2O2Se. Consequently, a high power factor of more than 400 µW m−1 K−2 over an unprecedented temperature range (80–200 K) is achieved, corresponding to the persistently high mobility arising from the highly gate‐tunable scattering mechanism. This finding provides a new avenue for maximizing thermoelectric performance by changing the scattering mechanism and carrier mobility over a wide temperature range.

Fang Yang, Jing Wu, Ady suwardi, YunShan Zhao, Boyuan Liang, Jie Jiang, Jianwei Xu, Dongzhi Chi, Kedar Hippalgaonkar, Junpeng Lu* and Zhenhua Ni*

2020 (InfoMat)

Ultrasensitive graphene‐Si position‐sensitive detector for motion tracking

Abstract:Motion tracking has attracted great attention in the fields of real‐time tracking, nanorobotics, and targeted therapy. For achieving more accurate motion tracking, the highly sensitive position‐sensitive detector (PSD) is desirable. Here, we demonstrate a meliorated PSD based on graphene‐Si heterojunction for motion tracking. The position sensitivity of PSD was improved by employing surface engineering of graphene. Through modulating the transport property of graphene, nearly 20‐fold increase of sensitivity was achieved under weak light, and at the same time, the detection limit power was reduced to ~ 2 nW. A motion tracking system was developed based on the improved PSD, and human arm swing was tracked, which demonstrated high sensitivity and real‐time tracking capabilities of the PSD. In addition, the PSD can support up to ~ 10 kHz high‐frequency tracking. This work provides a new strategy for improving the performance of PSD, and promotes the development of two‐dimensional materials in novel optoelectronic devices.

Wenhui Wang, Kaiyang Liu, Jie Jiang, Ruxia Du, Litao Sun, Wei Chen, Junpeng Lu and Zhenhua Ni*

2020 (Nano Research)

How defects influence the photoluminescence of TMDCs

Abstract:Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers, a class of ultrathin materials with a direct bandgap and high exciton binding energies, provide an ideal platform to study the photoluminescence (PL) of light-emitting devices. Atomically thin TMDCs usually contain various defects, which enrich the lattice structure and give rise to many intriguing properties. As the influences of defects can be either detrimental or beneficial, a comprehensive understanding of the internal mechanisms underlying defect behaviour is required for PL tailoring. Herein, recent advances in the defect influences on PL emission are summarized and discussed. Fundamental mechanisms are the focus of this review, such as radiative/nonradiative recombination kinetics and band structure modification. Both challenges and opportunities are present in the field of defect manipulation, and the exploration of mechanisms is expected to facilitate the applications of 2D TMDCs in the future.

Mengfan Zhou, Wenhui Wang, Junpeng Lu* and Zhenhua Ni*

2019 (Research)

Defect Engineering in 2D Materials: Precise Manipulation and Improved Functionalities

Abstract:Two-dimensional (2D) materials have attracted increasing interests in the last decade. The ultrathin feature of 2D materials makes them promising building blocks for next-generation electronic and optoelectronic devices. With reducing dimensionality from 3D to 2D, the inevitable defects will play more important roles in determining the properties of materials. In order to maximize the functionality of 2D materials, deep understanding and precise manipulation of the defects are indispensable. In the recent years, increasing research efforts have been made on the observation, understanding, manipulation, and control of defects in 2D materials. Here, we summarize the recent research progress of defect engineering on 2D materials. The defect engineering triggered by electron beam (e-beam), plasma, chemical treatment, and so forth is comprehensively reviewed. Firstly, e-beam irradiation-induced defect evolution, structural transformation, and novel structure fabrication are introduced. With the assistance of a high-resolution electron microscope, the dynamics of defect engineering can be visualized in situ. Subsequently, defect engineering employed to improve the performance of 2D devices by means of other methods of plasma, chemical, and ozone treatments is reviewed. At last, the challenges and opportunities of defect engineering on promoting the development of 2D materials are discussed. Through this review, we aim to build a correlation between defects and properties of 2D materials to support the design and optimization of high-performance electronic and optoelectronic devices.

Jie Jiang, Tao Xu, Junpeng Lu, Litao Sun* and Zhenhua Ni*

2019 (Applied Physics Letters)

Thermal transport and energy dissipation in two-dimensional Bi2O2Se

Abstract:Thermal transport and energy dissipation are important for a material in both thermoelectric and electronic devices. Here, we investigate the lateral and interfacial thermal transport of two-dimensional (2D) Bi2O2Se by Raman spectroscopy. It is found that thin Bi2O2Se flakes have a low in-plane thermal conductivity while maintaining an appropriate interfacial thermal conductance. The in-plane thermal conductivity of Bi2O2Se decreases with decreasing thickness, to as low as 0.92 ± 0.18 W⋅m−1⋅K−1 at a thickness of ∼8 nm. Such a low thermal conductivity is derived from the low phonon group velocity, strong anharmonicity, and large surface scattering of acoustic phonons of the Bi2O2Se thin layer. Simultaneously, thinner Bi2O2Se presents a higher thermal dissipation to the substrate than the thicker counterparts in the device. The interfacial thermal conductance increases with decreasing thickness, and reaches ∼21 MW⋅m−2⋅K−1 at ∼8 nm. These results provide critical information for the design of thermoelectric devices with high figures of merit and electronics with low-power consumption based on 2D materials.

Fang Yang, Ridong Wang, Weiwei Zhao, Jie Jiang, Xin Wei, Ting Zheng, Yutian Yang, Xinwei Wang, Junpeng Lu* and Zhenhua Ni*

2019 (Nano Letters)

Graphene-Based Infrared Position-Sensitive Detector for Precise Measurements and High-Speed Trajectory Tracking

Abstract:Noncontact optical sensing plays an important role in various applications, for example, motion tracking, pilotless automobile, precision machining, and laser radars. A device with features of high resolution, fast response, and safe detection (operation wavelength at infrared (IR)) is highly desired in such applications. Here, a near IR position-sensitive detector constructed by graphene-Ge Schottky heterojunction has been demonstrated. The device shows high responsivity (minimum detectable power of ∼10 nW), excellent spatial resolution (<1 μm), fast response time (∼μs), and could operate in a wide spectral range (from visible to ∼1600 nm). Applications of precise angle (∼5 × 10–6 degree) and vibration frequency (up to 10 kHz) measurements, as well as the trajectory tracking of a high-speed infrared target (∼100 km/h), have been realized based on this device. This work therefore provides a promising route for a high-performance noncontact IR optical sensing system.

Kaiyang Liu, Wenhui Wang, Yuanfang Yu, Xiangyu Hou, Yanpeng Liu, Wei Chen, Xiaomu Wang, Junpeng Lu* and Zhenhua Ni*

2019 (Advanced Materials)

Fast Photoelectric Conversion in the Near‐Infrared Enabled by Plasmon‐Induced Hot‐Electron Transfer

Abstract: Interfacial charge transfer is a fundamental and crucial process in photoelectric conversion. If charge transfer is not fast enough, carrier harvesting can compromise with competitive relaxation pathways, e.g., cooling, trapping, and recombination. Some of these processes can strongly affect the speed and efficiency of photoelectric conversion. In this work, it is elaborated that plasmon‐induced hot‐electron transfer (HET) from tungsten suboxide to graphene is a sufficiently fast process to prevent carrier cooling and trapping processes. A fast near‐infrared detector empowered by HET is demonstrated, and the response time is three orders of magnitude faster than that based on common band‐edge electron transfer. Moreover, HET can overcome the spectral limit of the bandgap of tungsten suboxide (≈2.8 eV) to extent the photoresponse to the communication band of 1550 nm (≈0.8 eV). These results indicate that plasmon‐induced HET is a new strategy for implementation of efficient and high‐speed photoelectric devices.

Yuanfang Yu, Yue Sun, Zhenliang Hu, Xuhong An, Dongming Zhou, Hongzhi Zhou, Wenhui Wang, Kaiyang Liu, Jie Jiang, Dandan Yang, Zainab Zafar, Haibo Zeng, Fengqiu Wang, Haiming Zhu, Junpeng Lu* and Zhenhua Ni*

2019 (Advanced Functional Materials)

Sulfur‐Mastery: Precise Synthesis of 2D Transition Metal Dichalcogenides

Abstract: Chemical vapor deposition (CVD) has been developed as the most promising method for the growth of transition metal dichalcogenides (TMDs). In this work, the key factor determining the growth of TMDs is ascertained. A straightforward method is devised to directly achieve a holistic control of thickness, shape, and size of WS2 flakes via a single parameter control, namely, the status of the S‐precursor. The thickness‐dependent growth of WS2 flakes from mono‐ to quad‐layers is achieved by precise control of the feeding rate of elemental S‐precursor. Moreover, the explicit control over amount and exposure time of S‐precursor determines the most optimum combination of these parameters to tune the shape of the crystals from triangular to hexagonal with appropriate size. Hence, the experimental findings provide a promising strategy to engineer the growth evolution of WS2 atomic layers by fine tuning of the sulfur supply, paving a pathway to scalable electronic and photonic devices.

Amina Zafar, Zainab Zafar, Weiwei Zhao, Jie Jiang, Yan Zhang, Yunfei Chen, Junpeng Lu* and Zhenhua Ni*

2018 (Advanced Materials)

Defect Engineering for Modulating the Trap States in Two-dimensional Photoconductor

Abstract: Defect induced trap states are essential in determining the performance of semiconductor photodetectors. The de-trap time of carriers from a deep trap could be prolonged by several orders of magnitude as compared to shallow trap, resulting in additional decay/response time of the device. Here, we demonstrate that the trap states in two-dimensional ReS2 could be efficiently modulated by defect engineering through molecule decoration. The deep traps that greatly prolong the response time could be mostly filled by Protoporphyrin (H2PP) molecules. At the same time, carrier recombination and shallow traps would in-turn play dominant roles in determining the decay time of the device, which can be several orders of magnitude faster than the as-prepared device. Moreover, the specific detectivity of the device is enhanced (as high as ~1.89×1013 Jones) due to the significant reduction of dark current through charge transfer between ReS2 and molecules. Defect engineering of trap states therefore provides a solution to achieve photodetectors with both high responsivity and fast response.

Jie Jiang#, Chongyi Ling#, Tao Xu, Wenhui Wang, Xianghong Niu, Amina Zafar, Zhenzhong Yan, Xiaomu Wang, Yumeng You, Litao Sun, Junpeng Lu, Jinlan Wang* and Zhenhua Ni*

2018 (Chemical Society Reviews)

Two-Dimensional Transitional Metal Dichalcogenides: Interface and Defect Engineering

Abstract: Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been considered as promising candidates for next generation nanoelectronics. Because of the atomically-thin structure and high surface to volume ratio, the interfaces involved in TMDCs-based devices play a predominant role in determining the device performance, such as charge injection/collection at metal/TMDCs interface, charge carrier trapping at the dielectric/TMDCs interface. On the other hand, the crystalline structures of TMDCs are enriched by a variety of intrinsic defects, including vacancies, adatoms, grain boundaries, and substitutional impurities. Customized design and engineering of the interfaces and defects provide an effective way to modulate the properties of TMDCs and finally enhance the device performances. Herein, we summarize and highlight recent advances and state-of-the-art investigation on the interface and defect engineering for TMDCs and the corresponding applications in electronic and optoelectronic devices. Various interface engineering approaches for TMDCs are overviewed, including surface charge transfer doping, TMDCs/metal contact engineering, and TMDCs/dielectric interface engineering. Subsequently, different types of structural defects in TMDCs are introduced. Defect engineering utilized to modulate the optical and electronic properties of TMDCs, as well as the developed high-performance and functional devices are summarized. Finally, we highlight the challenges and opportunities for interface and defect engineering in TMDCs materials for electronics and optoelectronics.

Zehua Hu#, Zhangting Wu#, Cheng Han#, Jun He, Zhenhua Ni*, Wei Chen*

2018 (Optica)

High performance position-sensitive-detector based on graphene-silicon heterojunction

Abstract: Position-sensitive-detectors (PSDs) based on lateral photoeffect have been widely used in diverse applications, including optical engineering, aerospace and military fields. With increasing demands in long working distance, low energy consumption, and weak signal sensing systems, the poor responsivity of conventional Silicon-based PSDs has become a bottleneck limiting their applications. Herein, we propose a high-performance passive PSD based on graphene-Si heterostructure. The graphene is adapted as a photon absorbing and charge separation layer working together with Si as a junction, while the high mobility provides promising ultra-long carrier diffusion length and facilitates large active area of the device. A PSD with working area of 8 mm × 8 mm is demonstrated to present excellent position sensitivity to weak light at nWs level (much better than the limit of ~μWs of Si p-i-n PSDs). More importantly, it shows very fast response and low degree of non-linearity of ~3%, and extends the operating wavelength to the near infrared (IR) region (1319 and 1550 nm). This work therefore provides a new strategy for high performance and broadband PSDs.

Wenhui Wang, Zhenzhong Yan, Jinfeng Zhang, Junpeng Lu, Hua Qin, and Zhenhua Ni*

2017 (Light:Science & Applications)

Interfacial amplification for graphene based position-sensitive-detectors

Abstract: Position-sensitive-detectors (PSDs) based on lateral photoeffect has been widely used in diverse applications, including optical engineering, aerospace and military fields. With increasing demand in long distance, low energy consumption, and weak signal sensing systems, the poor responsivity of conventional PSDs has become a bottleneck limiting its applications, e.g. silicon p-n or p-i-n junctions, or other materials and architectures. Herein, we present a high performance graphene based PSDs with revolutionary interfacial amplification mechanism. Signal amplification in the order of ~10^4 has been demonstrated by utilizing the ultrahigh mobility of graphene and long lifetime of photo-induced carriers at the interface of SiO2/Si. This would improve the detection limit of Si-based PSDs from uW to nW level, without sacrificing the spatial resolution and response speed. Such interfacial amplification mechanism is compatible with current Si technology and can be easily extended to other sensing systems.

Wen-Hui Wang, Ru-Xia Du, Xi-Tao Guo, Jie Jiang, Wei-Wei Zhao, Zhong-Hua Ni, Xin-Ran Wang, Yu-Meng You and Zhen-Hua Ni*

2017 (IEEE Electron Device Letters )

High-performance graphene-based electrostatic field sensor

Abstract: Electrostatic sensing technology is widely utilized in both military and civilian applications, including electrostatic prevention in gas stations and various electronic devices. The high sensitivity of electrostatic sensor is capable to detect not only weak electrostatic charges, but also the weak disturbance of electrostatic field in distant. Here, we present a high-performance graphene-based electrostatic sensor. Combining the ultrahigh mobility of graphene and the long lifetime of carriers in lightly doped SiO2/Si substrate, our device achieves a fast response of ~2 s and detection limit of electrostatic potential as low as ~5 V, which is improved by an order of magnitude as compared to commercial product. The proposed device structure opens a promising pathway to high-sensitive electrostatic detection, and also greatly facilitates the development of novel sensors, e.g. portable and flexible electrostatic sensor.

Wenhui Wang, Ruxia Du, Linping He, Weiwei Zhao, Yunfei Chen, Junpeng Lu and Zhenhua Ni*

2017 (Nano Research)

Probing the intrinsic optical quality of CVD grown MoS2

Abstract: Optical emission efficiency of two-dimensional layered transition metal dichalcogenides (TMDs) is one of the most important parameters affecting their optoelectronic performance. The optimization of the growth parameters by chemical vapor deposition (CVD) to achieve optoelectronic-grade quality TMDs is, therefore, highly desirable. Here, we present a systematic photoluminescence (PL) spectroscopic approach to assess the intrinsic optical and crystalline quality of CVD grown MoS2 (CVD MoS2). We propose the use of the intensity ratio between the PL measured in air and vacuum as an effective way to monitor the intrinsic optical quality of CVD MoS2. Low-temperature PL measurements are also used to evaluate the structural defects in MoS2, via defect-associated bound exciton emission, which well correlates with the field-effect carrier mobility of MoS2 grown at different temperatures. This work therefore provides a sensitive, noninvasive method to characterize the optical properties of TMDs, allowing the tuning of the growth parameters for the development of optoelectronic devices.

Amina Zafar,Haiyan Nan ,Zainab Zafar ,Zhangting , WuJie JiangYumeng , Zhenhua Ni

2016 (Optica)

High-performance graphene photodetector using interfacial gating

Abstract: Graphene-based photodetectors have recently received much attention for their potential to detect weak signals and their short response time, both of which are crucial in applications such as optical positioning, remote sensing, and biomedical imaging. However, existing devices for detecting weak signals are limited by the current photogating mechanism, so the price for achieving ultrahigh sensitivity is to sacrifice response time. In this work, we bridge the gap between ultrafast response and ultrahigh sensitivity by employing a graphene/SiO2/lightlygraphene/SiO2/lightly doped Si architecture with an interfacial gating mechanism. Our device is capable of detecting a signal of less than 1nW,and the spectral response extends from the visible to near-IR. More important, the photoresponse time of our device has been pushed .The current device structure does not need a complicated fabrication process and is fully compatible with silicon technology. This work not only will open up a route to graphene-based high-performance optoelectronic devices but also has great potential for ultrafast weak signal detection.

Xitao Guo, Wenhui Wang, Haiyan Nan, Yuanfang Yu, Jie Jiang, Weiwei Zhao, Jinhuan Li, Zainab Zafar, Nan Xiang, Zhonghua Ni, Weida Hu, Yumeng You, and Zhenhua Ni

2016-12 (Nano Research)

Defects as a factor limiting carrier mobility in WSe2: A spectroscopic investigation

Abstract: The electrical performance of two-dimensional transition metal dichalcogenides (TMDs) is strongly affected by the number of structural defects. In this work, we provide an optical spectroscopic characterization approach to correlate the number of structural defects and the electrical performance of WSe2 devices. Low-temperature photoluminescence (PL) spectra of electron-beam-lithographyprocessed WSe2 exhibit a clear defect-induced PL emission due to excitons bound to defects, which would strongly degrade the electrical performance. By adopting an electron-beam-free transfer-electrode technique, we successfully prepared a backgated WSe2 device containing a limited amount of defects. A maximum hole mobility of approximately 200 cm2·V–1·s–1 was achieved because of the reduced scattering sources, which is the highest reported value for this type of device. This work provides not only a versatile and nondestructive method to monitor the defects in TMDs but also a new route to approach the room-temperature phonon-limited mobility in high-performance TMD devices.

Zhangting Wu, Zhongzhong Luo, Yuting Shen, Weiwei Zhao, Wenhui Wang, Haiyan Nan, Xitao Guo, Litao Sun, Xinran Wang, Yumeng You,* and Zhenhua Ni*

2015 (Applied Physics Letters)

Distinct photoresponse in graphene induced by laser irradiation

Abstract: The graphene-based photodetector with tunable p-p+-p junctions was fabricated through a simple laser irradiation process. Distinct photoresponse was observed at the graphene (G)- laser irradiated graphene (LIG) junction by scanning photocurrent measurements, and its magnitude can be modulated as a result of a positive correlation between the photocurrent and doping concentration in LIG region. Detailed investigation suggests that the photo-thermoelectric effect, instead of the photovoltaic effect, dominates the photocurrent generation at the G-LIG junctions. Such a simple and low-cost technique offers an alternative way for the fabrication of graphene-based optoelectronic devices.

Wen Hui Wang, Hai Yan Nan, Qi Liu, Zheng Liang, Zhi Hao Yu, Feng Yuan Liu, Wei Da Hu, Wei Zhang, Xin Ran Wang, Zhen Hua Ni*

2014 (ACS Nano)

Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding

Abstract: We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro- PL and Raman images clearly reveal that the PL enhancement occurs at cracks/defects formed during high temperature vacuum annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include: (1) the oxygen chemical adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of non-radiative recombination of excitons at defect sites as verified by low temperature PL measurements. First principle calculations reveal a strong binding energy of ~2.395 eV for oxygen molecule adsorbed on an S vacancy of MoS2. The chemical adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to physical adsorbed oxygen on ideal MoS2 surface. We also demonstrate that the defect engineering and oxygen bonding could be easily realized by oxygen plasma irradiation. X-ray photoelectron spectroscopy further confirms the formation of Mo-O bonding. Our results provide a new route for modulating the optical properties of two dimensional semiconductors. The strong and stable PL from defects sites of MoS2 may have promising applications in optoelectronic devices.

Haiyan Nan, Zilu Wang, Wenhui Wang, Zheng Liang, Yan Lu, Qian Chen, Daowei He, Pingheng Tan, Feng Miao, Xinran Wang, Jinlan Wang* and Zhenhua Ni*

(2013) Materials Science and Engineering

Plasmons in graphene: Recent progress and applications

Abstract: Owing to its excellent electrical, mechanical, thermal and optical properties, graphene has attracted great interests since it was successfully exfoliated in 2004. Its two dimensional nature and superior properties meet the need of surface plasmons and greatly enrich the field of plasmonics. Recent progress and applications of graphene plasmonics will be reviewed, including the theoretical mechanisms, experimental observations, and meaningful applications. With relatively low loss, high confinement, flexible feature, and good tunability, graphene can be a promising plasmonic material alternative to the noble metals. Optics transformation, plasmonic metamaterials, light harvesting etc. are realized in graphene based devices, which are useful for applications in electronics, optics, energy storage, THz technology and so on. Moreover, the fine biocompatibility of graphene makes it a very well candidate for applications in biotechnology and medical science.

Luo XG, Qiu T*, Lu WB*, Ni ZH*

2013 (ACS Nano)

Layer-by-Layer Thinning of MoS2 by Plasma

Abstract: The thermal stability in air of graphene synthesized by either chemical vapor deposition or mechanical cleavage is studied. It is found that single layer graphene prepared by both methods starts to show defects at ~500 C, indicated by the appearance of a disorder-induced Raman D peak. The defects are initially sp3 type and become vacancy like at higher temperature. On the other hand, bilayer graphene shows better thermal stability, and the D peak appears at ~600 C. These results are quite different from those annealing in vacuum and controlled atmosphere. Raman images show that the defects in chemical vapor deposition graphene are not homogeneous, whereas those in mechanical cleavage graphene are uniformly distributed across the whole sample. The factors that affect the thermal stability of graphene are discussed. Our results could be important for guiding the future electronics process and chemical decoration of graphene.

Yulu Liu,,Haiyan Nan, Xing Wu, Wei Pan, Wenhui Wang,Jing Bai, Weiwei Zhao,Litao Sun, Xinran Wang, and Zhenhua Ni*