Robust n-type doping of WSe2 enabled by controllable proton irradiation
Nano Research 2023, 16 (1): 1220-1227
Published: 27 August 2022

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are considered to be promising building blocks for the next generation electronic and optoelectronic devices. Various doping schemes and work function engineering techniques have been explored to overcome the intrinsic performance limits of 2D TMDs. However, a reliable and long-time air stable doping scheme is still lacking in this field. In this work, we utilize keV ion beams of H2+ to irradiate layered WSe2 crystals and obtain efficient n-type doping effect for all irradiated crystals within a fluence of 1 × 1014 protons·cm−2 (1e14). Moreover, the irradiated WSe2 remains an n-type semiconductor even after it is exposed to ambient conditions for a year. Localized ion irradiation with a focused beam can directly pattern on the sample to make high performance homogenous p-n junction diodes. Raman and photoluminescence (PL) spectra demonstrate that the WSe2 crystal lattice stays intact after irradiation within 1e14. We attribute the reliable electron-doping to the significant increase in Se vacancies after the proton irradiation, which is confirmed by our scanning transmission electron microscope (STEM) results. Our work demonstrates a reliable and long-term air stable n-type doping scheme to realize high-performance electronic TMD devices, which is also suitable for further integration with other 2D devices.

Research Article Issue
Two-dimensional reconfigurable electronics enabled by asymmetric floating gate
Nano Research 2022, 15 (5): 4439-4447
Published: 29 January 2022

Reconfigurable devices with customized functionalities hold great potential in addressing the scaling limits of silicon-based field-effect transistors (FETs). The conventional reconfigurable FETs are limited to the applications in logic circuits, and the commonly used multi-gate programming strategies often lead to high power consumption and device complexity. Here, we report a reconfigurable WSe2 optoelectronic device that can function as photodiode, artificial synapse, and 2-bit memory in a single device, enabled by an asymmetric floating gate (AFG) that can continuously program the device into different homojunction modes. The lateral p−n homojunction formed in the AFG device exhibits high-performance self-powered photodetection, with a responsivity over 0.17 A·W−1 and a wide detection spectral range from violet to near-infrared region. The AFG device can also mimic synaptic features of biological synapses and achieve distinct potentiation/depression behaviors under the modulation of both drain-source bias and light illumination. Moreover, when working as a 2-bit memory via the transition between n−n+ and p−n homojunctions, the AFG device shows four distinct conductive states with a high on/off current ratio over 106 and good repeatability. Combining reduced processing complexity and reconfigurable functionalities, the WSe2 AFG devices demonstrate great potential towards high-performance photoelectric interconnected circuits.

Research Article Issue
Controlling phase transition in WSe2 towards ideal n-type transistor
Nano Research 2021, 14 (8): 2703-2710
Published: 04 January 2021

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been rapidly established as promising building blocks for versatile atomic scale circuits and multifunctional devices. However, the high contact resistance in TMDs based transistors seriously hinders their applications in complementary electronics. In this work, we show that an Ohmic homojunction n-type tungsten diselenide (WSe2) transistor is realized through spatially controlling cesium (Cs) doping region near the contacts. We find that the remarkable electron doping effect of Cs stimulates a semiconductor to metal (2H to 1T’) phase transition in WSe2, and hence the formation of 2H-1T’ hetero-phase contact. Our method significantly optimizes the WSe2 transport behavior with a perfect low subthreshold swing of ~ 61 mV/dec and ultrahigh current on/off ratio exceeding ~ 109. Meanwhile, the electron mobility is enhanced by nearly 50 times. We elucidate that the ideal n-type behavior originates from the negligible Schottky barrier height of ~ 19 meV and low contact resistance of ~ 0.9 kΩ·μm in the 2H-1T’ homojunction device. Moreover, based on the Ohmic hetero-phase configuration, a WSe2 inverter is achieved with a high gain of ~ 270 and low power consumption of ~ 28 pW. Our findings envision Cs functionalization as an effective method to realize ideal Ohmic contact in 2D WSe2 transistors towards high performance complementary electronic devices.

Research Article Issue
An in-situ spectroscopy investigation of alkali metal interaction mechanism with the imide functional group
Nano Research 2020, 13 (12): 3224-3229
Published: 12 August 2020

Organic anode materials have attracted considerable interest owing to their high tunability by adopting various active functional groups. However, the interaction mechanisms between the alkali metals and the active functional groups in host materials have been rarely studied systematically. Here, a widely used organic semiconductor of perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) was selected as a model system to investigate how alkali metals interact with imide functional groups and induce changes in chemical and electronic structures of PTCDI. The interaction at the alkali/PTCDI interface was probed by in-situ x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), synchrotron-based near edge X-ray absorption fine structure (NEXAFS), and corroborated by density functional theory (DFT) calculations. Our results indicate that the alkali metal replaces the hydrogen atoms in the imide group and interact with the imide nitrogen of PTCDI. Electron transfer induced gap states and downward band-bending like effects are identified on the alkali-deposited PTCDI surface. It was found that Na shows a stronger electron transfer effect than Li. Such a model study of alkali insertion/intercalation in PTCDI gives insights for the exploration of the potential host materials for alkali storage applications.

Review Article Issue
Surface charge transfer doping for two-dimensional semiconductor- based electronic and optoelectronic devices
Nano Research 2021, 14 (6): 1682-1697
Published: 10 July 2020

Doping of semiconductors, i.e., accurately modulating the charge carrier type and concentration in a controllable manner, is a key technology foundation for modern electronics and optoelectronics. However, the conventional doping technologies widely utilized in silicon industry, such as ion implantation and thermal diffusion, always fail when applied to two-dimensional (2D) materials with atomically-thin nature. Surface charge transfer doping (SCTD) is emerging as an effective and non-destructive doping technique to provide reliable doping capability for 2D materials, in particular 2D semiconductors. Herein, we summarize the recent advances and developments on the SCTD of 2D semiconductors and its application in electronic and optoelectronic devices. The underlying mechanism of STCD processes on 2D semiconductors is briefly introduced. Its impact on tuning the fundamental properties of various 2D systems is highlighted. We particularly emphasize on the SCTD-enabled high-performance 2D functional devices. Finally, the challenges and opportunities for the future development of SCTD are discussed.

Research Article Issue
Single-molecule imaging of dinitrogen molecule adsorption on individual iron phthalocyanine
Nano Research 2020, 13 (9): 2393-2398
Published: 16 June 2020

Nitrogen fixation is a vital process for both nature and industry. Whereas the nitrogenase can reduce nitrogen in ambient environment in nature, the industrialized Haber-Bosch process is a high temperature and high-pressure process. Since the discovery of the first dinitrogen complex in 1965, many dinitrogen complexes are prepared in a homogeneous solution to mimic the nitrogenase enzyme in nature. However, studies of the heterogeneous process on surface are rarely addressed. Moreover, molecular scale characterization for such dinitrogen complex is lacking. Here, we present a simple model system to investigate, at the single-molecule level, the binding of dinitrogen on a surface confined iron phthalocyanine (FePc) monolayer through the combination of in-situ low-temperature scanning tunneling microscopy (LT-STM) and x-ray photoelectron spectroscopy (XPS) measurements. The iron center in FePc molecule deposited on Au(111) and highly oriented pyrolytic graphite (HOPG) surface can adsorb dinitrogen molecule at room temperature and low pressure. A comparative study reveals that the adsorption behaviors of FePc on these two different substrates are identical. Chemical bond is formed between the dinitrogen and the Fe atom in the FePc molecule, which greatly modifies the electronic structure of FePc. The bonding is reversible and can be manipulated by applying bias using a STM tip or by thermal annealing.

Research Article Issue
Black phosphorus inverter devices enabled by in-situ aluminum surface modification
Nano Research 2019, 12 (3): 531-536
Published: 21 November 2018

Two-dimensional black phosphorus (BP) generally exhibits a hole-dominated transport characteristic when configured as field-effect transistor devices. The effective control of charge carrier type and concentration is very crucial for the application of BP in complementary electronics. Herein, we report a facile and effective electron doping methodology on BP, through in situ surface modification with aluminum (Al). The electron mobility of few-layer BP is found to be largely enhanced to ~ 10.6 cm2·V-1·s-1 by over 6 times after aluminum modification. In situ photoelectron spectroscopy characterization reveals the formation of Al-P covalent bond at the interface, which can also serve as local gate to tune the transport properties in BP layers. Finally, a spatially-controlled aluminum doping technique is employed to establish a p-n homojunction on a single BP flake, and hence to realize the complementary inverter devices, where the highest gain value of ~ 33 is obtained.

Research Article Issue
Significantly enhanced optoelectronic performance of tungsten diselenide phototransistor via surface functionalization
Nano Research 2017, 10 (4): 1282-1291
Published: 06 January 2017

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted enormous research interests and efforts towards the development of versatile electronic and optical devices, owing to their extraordinary and unique fundamental properties and remarkable prospects in nanoelectronic applications. Among the TMDs, tungsten diselenide (WSe2) exhibits tunable ambipolar transport characteristics and superior optical properties such as high quantum efficiency. Herein, we demonstrate significant enhancement in the device performance of WSe2 phototransistor by in situ surface functionalization with cesium carbonate (Cs2CO3). WSe2 was found to be strongly doped with electrons after Cs2CO3 modification. The electron mobility of WSe2 increased by almost one order of magnitude after surface functionalization with 1.6-nm-thick Cs2CO3 decoration. Furthermore, the photocurrent of the WSe2-based phototransistor increased by nearly three orders of magnitude with the deposition of 1.6-nm-thick Cs2CO3. Characterizations by in situ photoelectron spectroscopy techniques confirmed the significant surface charge transfer occurring at the Cs2CO3/WSe2 interface. Our findings coupled with the tunable nature of the surface transfer doping method establish WSe2 as a promising candidate for future 2D materials- based optoelectronic devices.

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