Zhangting Wu

Defect-Engineered Heat Transport in Graphene: A Route to High Efficient Thermal Rectification.

Low-dimensional materials such as graphene provide an ideal platform to probe the correlation between thermal transport and lattice defects, which could be engineered at the molecular level. In this work, we perform molecular dynamics simulations and non-contact optothermal Raman measurements to study this correlation. We find that oxygen plasma treatment could reduce the thermal conductivity of graphene significantly even at extremely low defect concentration (∼83% reduction for ∼0.1% defects), which could be attributed mainly to the creation of carbonyl pair defects. Other types of defects such as hydroxyl, epoxy groups and nano-holes demonstrate much weaker effects on the reduction where the sp2 nature of graphene is better preserved. With the capability of selectively functionalizing graphene, we propose an asymmetric junction between graphene and defective graphene with a high thermal rectification ratio of ∼46%, as demonstrated by our molecular dynamics simulation results. Our findings provide fundamental insights into the physics of thermal transport in defective graphene, and two-dimensional materials in general, which could help on the future design of functional applications such as optothermal and electrothermal devices.

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

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.

Probing the intrinsic optical quality of CVD grown MoS2

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.

Ultrafast Growth of High‐Quality Monolayer WSe2 on Au

The ultrafast growth of high-quality uniform monolayer WSe2 is reported with a growth rate of ≈26 µm s-1 by chemical vapor deposition on reusable Au substrate, which is ≈2-3 orders of magnitude faster than those of most 2D transition metal dichalcogenides grown on nonmetal substrates. Such ultrafast growth allows for the fabrication of millimeter-size single-crystal WSe2 domains in ≈30 s and large-area continuous films in ≈60 s. Importantly, the ultrafast grown WSe2 shows excellent crystal quality and extraordinary electrical performance comparable to those of the mechanically exfoliated samples, with a high mobility up to ≈143 cm2 V-1 s-1 and ON/OFF ratio up to 9 × 106 at room temperature. Density functional theory calculations reveal that the ultrafast growth of WSe2 is due to the small energy barriers and exothermic characteristic for the diffusion and attachment of W and Se on the edges of WSe2 on Au substrate.

Synthesis, Optical, and Magnetic Properties of Ba2Ni3F10 Nanowires

A low-temperature hydrothermal route has been developed, and pure phase Ba2Ni3F10 nanowires have been successfully prepared under optimized conditions. Under the 325 nm excitation, the Ba2Ni3F10 nanowires exhibit three emission bands with peak positions locating at 360, 530, and 700 nm, respectively. Combined with the first-principles calculations, the photoluminescence property can be explained by the electron transitions between the t2g and eg orbitals. Clear hysteresis loops observed below the temperature of 60 K demonstrates the weak ferromagnetism in Ba2Ni3F10 nanowires, which has been attributed to the surface strain of nanowires. Exchange bias with blocking temperature of 55 K has been observed, which originates from the magnetization pinning under the cooling field due to antiferromagnetic core/weak ferromagnetic shell structure of Ba2Ni3F10 nanowires.

Low-Temperature Eutectic Synthesis of PtTe2 with Weak Antilocalization and Controlled Layer Thinning

Metallic transition metal dichalcogenides (TMDs) have exhibited various exotic physical properties and hold the promise of novel optoelectronic and topological devices applications. However, the synthesis of metallic TMDs is based on gas-phase methods and requires high temperature condition. As an alternative to the gas-phase synthetic approach, lower temperature eutectic liquid-phase synthesis presents a very promising approach with the potential for larger-scale and controllable growth of high-quality thin metallic TMDs single crystals. Herein, we report the first realization of low-temperature eutectic liquid-phase synthesis of type-II Dirac semimetal PtTe2 single crystals with thickness ranging from 2 to 200 nm. The electrical measurement of synthesized PtTe2 reveals a record-high conductivity of as high as 3.3*106 S/m at room temperature. Besides, we experimentally identify the weak antilocalization behavior in the type-II Dirac semimetal PtTe2 for the first time. Furthermore, we develop a simple and general strategy to obtain atomically-thin PtTe2 crystal by thinning as-synthesized bulk samples, which can still retain highly crystalline and exhibits excellent electric conductivity. Our results of controllable and scalable low-temperature eutectic liquid-phase synthesis and layer-by-layer thinning of high-quality thin PtTe2 single crystals offer a simple and general approach for obtaining different thickness metallic TMDs with high-melting point transition metal.

Layer-number dependent and structural defect related optical properties of InSe

Two-dimensional (2D) InSe is an attractive semiconductor because of its bandgap in the near infrared region, high carrier mobility and chemical stability. Here, we present systematic investigations on the layer-dependent optical properties of few-layer InSe (2–6 layers). We develop a quantitative calibration map using optical images and Raman and photoluminescence (PL) spectroscopy to directly identify the layer numbers of the InSe flakes. This is facilitated by the significant difference in the optical contrast or Raman/PL spectra of InSe with different thicknesses. Moreover, excitonic states in few-layer InSe and in the bulk are probed by temperature-dependent PL spectroscopy. An emerging peak arising from the recombination of excitons bound to localized states is observed at low temperature. These states arise from structural defects which can also be induced via electron beam irradiation. The PL emission from bound excitons could be modified to be even stronger than the emission from near band edge recombination. This provides a new strategy to improve the PL emission efficiency of 2D InSe.