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Chirality arises from the asymmetry of matters, where two counterparts are the mirror image of each other. The interaction between circular-polarization light and quantum materials is enhanced in chiral space groups due to the structural chirality. Tellurium (Te) possesses the simplest chiral crystal structure, with Te atoms covalently bonded into a spiral atomic chain (left- or right-handed) with a periodicity of three. Here, we investigate the tunable circular photo-electric responses in 2D Te field-effect transistor with different chirality, including the longitudinal circular photogalvanic effect induced by the radial spin texture (electron-spin polarization parallel to the electron momentum direction) and the circular photovoltaic induced by the chiral crystal structure (helical Te atomic chains). Our work demonstrates the controllable manipulation of the chirality degree of freedom in materials., Comment: 30 pages

The lattice thermal conductivity ($\kappa$) of two ceramic materials, cerium dioxide (CeO$_2$) and magnesium oxide (MgO), is computed up to 1500 K using first principles and the phonon Boltzmann Transport Equation (PBTE) and compared to time-domain thermoreflectance (TDTR) measurements up to 800 K. Phonon renormalization and the four-phonon effect, along with high temperature thermal expansion, are integrated in our \textit{ab initio} molecular dynamics (AIMD) calculations. This is done by first relaxing structures and then fitting to a set of effective force constants employed in a temperature-dependent effective potential (TDEP) method. Both three-phonon and four-phonon scattering rates are computed based on these effective force constants. Our calculated thermal conductivities from the PBTE solver agree well with literature and our TDTR measurements. Other predicted thermal properties including thermal expansion, frequency shift, and phonon linewidth also compare well with available experimental data. Our results show that high temperature softens phonon frequency and reduces four-phonon scattering strength in both ceramics. Compared to MgO, we find that CeO$_2$ has weaker four-phonon effect and renormalization greatly reduces its four-phonon scattering rates.

Inverse design has greatly expanded nanophotonic devices and brought optimized performance. However, the use of inverse design for plasmonic structures has been challenging due to local field concentrations that can lead to errors in gradient calculation when the continuum adjoint method is used. On the other hand, with the discrete adjoint method one can achieve the exact gradient. Historically the discrete version is exclusively used with a Finite Element model, and applying the Finite-Difference Time-Domain (FDTD) method in inverse design of plasmonic structures is rarely attempted. Due to the popularity of using FDTD in simulating plasmonic structures, we integrate the discrete adjoint method with FDTD and present a framework to carry out inverse design of plasmonic structures using density-based topology optimization. We demonstrate the exactness of the gradient calculation for a plasmonic block structure with varying permittivity. Another challenge that is unique with plasmonic structures is that non-physical amplification caused by poorly chosen material interpolation can destroy a stable convergence of the optimization. To avoid this, we adopt a non-linear material interpolation scheme in the FDTD solver. In addition, filtering-and-projection regularization is incorporated to ensure manufacturability of the designed structures. As an example of this framework, successful reconstruction of electric fields of a plasmonic bowtie aperture is presented., Comment: 43 pages, 18 figures

Tellurium can form nanowires of helical atomic chains. Given their unique one-dimensional van der Waals structure, these nanowires are expected to show remarkably different physical and electronic properties than bulk tellurium. Here we show that few-chain and single-chain van der Waals tellurium nanowires can be isolated using carbon nanotube and boron nitride nanotube encapsulation. With the approach, the number of atomic chains can be controlled by the inner diameter of the nanotube. The Raman response of the structures suggests that the interaction between a single-atomic tellurium chain and a carbon nanotube is weak, and that the inter-chain interaction becomes stronger as the number of chains increases. Compared with bare tellurium nanowires on SiO2, nanowires encapsulated in boron nitride nanotubes exhibit a dramatically enhanced current-carrying capacity, with a current density of 1.5*10^8 A cm-2, which exceeds that of most semiconducting nanowires. We also use our tellurium nanowires encapsulated in boron nitride nanotubes to create field-effect transistors that have a diameter of only 2 nm., Comment: 45 pages, 23 figures. Nature Electronics, to be published

Tellurium (Te) is an intrinsically p-type doped narrow bandgap semiconductor with excellent electrical conductivity and low thermal conductivity. Bulk trigonal Te has been theoretically predicted and experimentally demonstrated to be an outstanding thermoelectric material with high value of thermoelectric figure-of-merit ZT. In view of the recent progress in developing synthesis route of two-dimensional (2D) tellurium thin films as well as the growing trend of exploiting nanostructures as thermoelectric devices, here for the first time we report excellent thermoelectric performance of tellurium nanofilms, with room temperature power factor of 31.7 {\mu}Wcm-1K-2 and ZT value of 0.63. To further enhance the efficiency of harvesting thermoelectric power in nanofilm devices, thermoelectrical current mapping was performed with a laser as a heating source, and we found high work function metals such as palladium can form rare accumulation-type metal-to-semiconductor contacts to 2D Te, which allows thermoelectrically generated carriers to be collected more efficiently. High-performance thermoelectric 2D Te devices have broad applications as energy harvesting devices or nanoscale Peltier coolers in microsystems., Comment: manuscript+SI, 30 pages

Selenium has attracted intensive attention as a promising material candidate for future optoelectronic applications. However, selenium has a strong tendency to grow into nanowire forms due to its anisotropic atomic structure, which has largely hindered the exploration of its potential applications. In this work, using a physical vapor deposition method, we have demonstrated the synthesis of large-size, high-quality 2D selenium nanosheets, the minimum thickness of which could be as thin as 5 nm. The Se nanosheet exhibits a strong in-plane anisotropic property, which is determined by angle-resolved Raman spectroscopy. Back-gating field-effect transistors based on a Se nanosheet exhibit p-type transport behaviors with on-state current density around 20 mA/mm at Vds = 3 V. Four-terminal field effect devices are also fabricated to evaluate the intrinsic hole mobility of the selenium nanosheet, and the value is determined to be 0.26 cm2 Vs at 300 K. The selenium nanosheet phototransistors show an excellent photoresponsivity of up to 263 A/W, with a rise time of 0.1 s and fall time of 0.12 s. These results suggest that crystal selenium as a 2D form of a 1D van der Waals solid opens up the possibility to explore device applications., Comment: ACS Nano, 2017, 11 (10), pp 10222

Experimental demonstrations of 1D van der Waals material tellurium have been presented by Raman spectroscopy under strain and magneto-transport. Raman spectroscopy measurements have been performed under strains along different principle axes. Pronounced strain response along c-axis is observed due to the strong intra-chain covalent bonds, while no strain response is obtained along a-axis due to the weak inter-chain van der Waals interaction. Magneto-transport results further verify its anisotropic property, resulting in dramatically distinct magneto-resistance behaviors in terms of three different magnetic field directions. Specifically, phase coherence length extracted from weak antilocalization effect, L$_{\Phi}$ ~ T$^{-0.5}$, claims its 2D transport characteristics when an applied magnetic field is perpendicular to the thin film. In contrast, L$_{\Phi}$ ~ T$^{-0.33}$ is obtained from universal conductance fluctuations once the magnetic field is along c-axis of Te, indicating its nature of 1D transport along the helical atomic chains. Our studies, which are obtained on high quality single crystal tellurium thin film, appear to serve as strong evidences of its 1D van der Waals structure from experimental perspectives. It is the aim of this paper to address this special concept that differs from the previous well-studied 1D nanowires or 2D van der Waals materials.

The reliable production of two-dimensional crystals is essential for the development of new technologies based on 2D materials. However, current synthesis methods suffer from a variety of drawbacks, including limitations in crystal size and stability. Here, we report the fabrication of large-area, high-quality 2D tellurium (tellurene) using a substrate-free solution process. Our approach can create crystals with a process-tunable thickness, from monolayer to tens of nanometres, and with lateral sizes of up to 100 um. The chiral-chain van der Waals structure of tellurene gives rise to strong in-plane anisotropic properties and large thickness dependent shifts in Raman vibrational modes, which is not observed in other 2D layered materials. We also fabricate tellurene field-effect transistors, which exhibit air-stable performance at room temperature for over two months, on off ratios on the order of 106 and field-effect mobilities of around 700 cm2 per Vs. Furthermore, by scaling down the channel length and integrating with high-k dielectrics, transistors with a significant on-state current density of 1 A mm-1 are demonstrated.

Black phosphorus is emerging as a promising semiconductor for electronic and optoelectronic applications. To study fundamental carrier properties, we performed ultrafast femtosecond pump-probe spectroscopy on thin film black phosphorus mechanically exfoliated on a glass substrate. Carriers (electrons and holes) were excited to high energy levels and the process of carrier relaxation through phonon emission and recombination was probed. We used a wide range of probing wavelengths up to and across the band gap to study the evolution of the relaxation dynamics at different energy levels. Our experiments revealed a plethora of important physical phenomena. The fast relaxation time constants, associated with carrier-phonon scattering, steadily increase as the energy of the probe beam approaches the band gap energy, which was determined to be 0.31 eV, and the carrier recombination rate was obtained when the probe wavelength was tuned to match the band gap energy. The carrier-phonon scattering rates were found to be similar along the armchair and zigzag directions, therefore, the anisotropic carrier mobility reported in literature is mainly due to the difference in effective mass of carriers along different directions. The ultrafast spectroscopy data further revealed the oxidation induced surface charges. Our results highlight the importance of using the spectroscopy technique, in this case, in the mid-IR range, to uncover useful physical processes.

Topological insulators (TI) have attracted extensive research effort due to their insulating bulk states but conducting surface states. However, investigation and understanding of thermal transport in topological insulators, particularly the effect of surface states are lacking. In this work, we studied thickness-dependent in-plane thermal conductivity of Bi2Te2Se TI thin films. A large enhancement of both thermal and electrical conductivity was observed for films with thicknesses below 20 nm, which is attributed to the surface states and bulk-insulating nature of these films. Surprisingly, a surface Lorenz number of over 10 times the Sommerfeld value was found. Transport measurements indicated that the surface is near the charge neutrality point when the film thickness is below 20 nm. Possible reasons for the large Lorenz number include the electrical and thermal current decoupling in the surface state Dirac fluid and the bipolar diffusion transport involving surface states.

The Poisson's ratio of a material characterizes its response to uniaxial strain. Materials normally possess a positive Poisson's ratio - they contract laterally when stretched, and expand laterally when compressed. A negative Poisson's ratio is theoretically permissible but has not, with few exceptions of man-made bulk structures, been experimentally observed in any natural materials. Here, we show that the negative Poisson's ratio exists in the low-dimensional natural material black phosphorus, and that our experimental observations are consistent with first principles simulations. Through application of uniaxial strain along zigzag and armchair directions, we find that both interlayer and intralayer negative Poisson's ratios can be obtained in black phosphorus. The phenomenon originates from the puckered structure of its in-plane lattice, together with coupled hinge-like bonding configurations.

Transition metal pentatelluride ZrTe5 is a versatile material in condensed-matter physics and has been intensively studied since the 1980s. The most fascinating feature of ZrTe5 is that it is a 3D Dirac semimetal which has linear energy dispersion in all three dimensions in momentum space. Structure-wise, ZrTe5 is a layered material held together by weak interlayer van der Waals force. The combination of its unique band structure and 2D atomic structure provides a fertile ground for more potential exotic physical phenomena in ZrTe5 related to 3D Dirac semimentals. However the physical properties of its few-layer form have yet to be thoroughly explored. Here we report strong optical and electrical in-plane anisotropy of mechanically exfoliated few-layer ZrTe5. Raman spectroscopy shows significant intensity change with sample orientations, and the behavior of angle-resolved phonon modes at the gamma point is explained by theoretical calculation. DC conductance measurement indicates a 50% of difference along different in-plane directions. The diminishing of resistivity anomaly in few-layer samples indicates the evolution of band structure with reduced thickness. Low-temperature Hall experiment sheds lights on more intrinsic anisotropic electrical transport, with hole mobility of 3,000 and 1,500 cm2/Vs along a-axis and c-axis respectively. Pronounced quantum oscillations in magneto-resistance are observed at low temperatures with highest electron mobility up to 44,000 cm2/Vs.

The metal contacts on 2D black phosphorus field-effect transistor and photodetectors are studied. The metal work functions can significantly impact the Schottky barrier at the metal-semiconductor contact in black phosphorus devices. Higher metal work functions lead to larger output hole currents in p-type transistors, while ambipolar characteristics can be observed with lower work function metals. Photodetectors with record high photoresponsivity (223 mA/W) are demonstrated on black phosphorus through contact-engineering., Comment: 4 Pages

Black phosphorus has been revisited recently as a new two-dimensional material showing potential applications in electronics and optoelectronics. Here we report the anisotropic in-plane thermal conductivity of suspended few-layer black phosphorus measured by micro-Raman spectroscopy. The armchair and zigzag thermal conductivities are ~20 and ~40 W m$^{-1}$ K$^{-1}$ for black phosphorus films thicker than 15 nm, respectively, and decrease to ~10 and ~20 W m$^{-1}$ K$^{-1}$ as the film thickness is reduced, exhibiting significant anisotropy. The thermal conductivity anisotropic ratio is found to be ~2 for thick black phosphorus films and drops to ~1.5 for the thinnest 9.5-nm-thick film. Theoretical modeling reveals that the observed anisotropy is primarily related to the anisotropic phonon dispersion, whereas the intrinsic phonon scattering rates are found to be similar along the armchair and zigzag directions. Surface scattering in the black phosphorus films is shown to strongly suppress the contribution of long-mean-free-path acoustic phonons., Comment: Accepted by Nature Communications

Phosphorene, an elemental 2D material, which is the monolayer of black phosphorus, has been mechanically exfoliated recently. In its bulk form, black phosphorus shows high carrier mobility (~10000 cm2/Vs) and a ~0.3 eV direct bandgap. Well-behaved p-type field-effect transistors with mobilities of up to 1000 cm2/Vs, as well as phototransistors, have been demonstrated on few-layer black phosphorus, showing its promise for electronics and optoelectronics applications due to its high hole mobility and thickness-dependence direct bandgap. However, p-n junctions, the basic building blocks of modern electronic and optoelectronic devices, have not yet been realized based on black phosphorus. In this paper, we demonstrate a gate tunable p-n diode based on a p-type black phosphorus/n-type monolayer MoS2 van der Waals p-n heterojunction. Upon illumination, these ultra-thin p-n diodes show a maximum photodetection responsivity of 418 mA/W at the wavelength of 633 nm, and photovoltaic energy conversion with an external quantum efficiency of 0.3%. These p-n diodes show promise for broadband photodetection and solar energy harvesting., Comment: 37 pages

Transistors based on various types of non-silicon nanowires have shown great potential for a variety of applications, especially for those require transparency and low-temperature substrates. However, critical requirements for circuit functionality such as saturated source-drain current, and matched threshold voltages of individual nanowire transistors in a way that is compatible with low temperature substrates, have not been achieved. Here we show that femtosecond laser pulses can anneal individual transistors based on In2O3 nanowires, improve the saturation of the source-drain current, and permanently shift the threshold voltage to the positive direction. We applied this technique and successfully shifted the switching threshold voltages of NMOS based inverters and improved their noise margin, in both depletion and enhancement modes. Our demonstration provides a method to trim the parameters of individual nanowire transistors, and suggests potential for large-scale integration of nanowire-based circuit blocks and systems., Comment: 18 pages, 4 figures