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Both single layer Ga2O3 (SLGO) and graphene are attractive due to their respective electronic and mechanical properties such as wide bandgap and high electrical conductivity. Bringing them together by using van der Waals force to form a heterogeneous structure is new and worth to investigate. In this work, density functional theory (DFT) study of SLGO/graphene has been conducted through varying the interlayer distance. Standard procedures of calculating double-layer heterostructures using DFT has been followed, and several interesting phenomena have been unveiled, for example, band opening in conduction bands of the SLGO and graphene and switching effect of the in-plane optical absorption., Comment: 13 pages, 7 figures

Modern semiconductor materials are increasingly used in multidisciplinary systems demonstrating cross-interactions between mechanical strains and electronic potentials, which gives rise to ubiquitous applications in high sensitivity, self-powered sensor devices. One of fundamental prerequisites for such semiconductor materials to exhibit piezoelectric properties is the noncentrosymmetry of the crystal structures. $\beta$-Ga$_2$O$_3$ has been an emerging compound semiconductor material due to its ultra-wide bandgap. However the pristine $\beta$-Ga$_2$O$_3$ has an inversion center, displaying no piezoelectric effect. This work discovered that substitutionally doped $\beta$-Ga$_2$O$_3$ possesses piezoelectric property by using first principles method, while majority of previous research on its substitutional doping has been focusing on the purposes of increasing electrical conductivity and formation of the semiconductor heterojunctions. More interestingly, it is unveiled from this work that the formation energy has a clear relation with the piezoelectric coefficient., Comment: 11 pages, 6 figures

We investigate the thermoelectric performance of armchair graphene nanoribbon (AGNR), bilayer GNRs junction (BGNRJ) and BGNRJ with holes (BGNRJ-H) by the first principles calculation with non-equilibrium Green function. It is found that the BGNRJ-H exhibits high ZT values of 9.65 and 5.55 at 300K. The reason of these significantly larger ZT values than previously observed has been calculated due to reduced thermal conductivity and enhanced electrical conductivity. The low thermal conductance comes from the van der waals (vdW) interaction between two graphene layers. The increased electrical conductivity can be attributed to the coupling effect of aligned holes in both layers. It is found from analysis results that the electron transmission of the BGNRJ-H is much stronger than a normal BGNRJ, which gives rise to the higher electrical conductance and outstanding ZT values., Comment: 14 pages, 7 figures

Having simultaneously a high quality factor (i.e. a narrow resonant band) and a shorter interaction time between the resonating system and the external sources (i.e. a wide resonant band) is a desirable characteristic for mechanical resonators, which however has been regarded as contradictory. This has been known as the limit of Lorentz reciprocity. We explore a configuration to achieve this desired characteristic within the mechanical regime. The configuration consists of a pair of mechanical resonators coupled together through their connecting part. One of them is encapsulated in a vacuum environment, and the other is left in the normal ambient condition. Numerical model of this configuration shows clearly the advantages such as: (a), sensitivity to the change of resonant frequency is greatly improved (the product of bandwidth $\Delta \omega$ and the interaction time $\Delta t$ has increased at least two orders of magnitude); (b), the value of $\Delta \omega \cdot \Delta t$ can be adjusted through the coupling stiffness., Comment: 8 figures

We investigate the electron transport and thermoelectric property of twisted bilayer graphene nanoribbon junction (TBGNRJ) in $0^o$, $21.8^o$, $38.2^o$ and $60^o$ rotation angles by first principles calculation with Landauer-Buttiker and Boltzmann theories. It is found that TBGNRJs exhibit negative differential resistance (NDR) in $21.8^o$ and $38.2^o$ rotation angles under $\pm$ 0.2 V bias voltage. More importantly, three peak ZT values of 2.0, 2.7 and 6.1 can be achieved in the $21.8^o$ rotation angle at 300K. The outstanding ZT values of TBGNRJs are interpreted as the combination of the reduced thermal conductivity and enhanced electrical conductivity at optimized angles., Comment: 13 pages, 8 figures

Dynamical theories for piezotronic and piezo-phototropic devices are incomplete. In this work, we aim to establish a theoretical method for modelling dynamic characteristics of devices exhibiting these two emerging phenomena. By taking the simplest piezotronic device-PN junction as an example, we develop a small signal model and the united approach to analyze its diffusion capacitance and conductance under both low and high frequency external compressive stresses, which is different from the traditional considerations that treat the piezopotential as a static value. Furthermore, we expand the theory into piezo-phototronic devices e.g., a light emitting diode (LED). The dynamic recombination rate and light emitting intensity are quantitatively calculated under different frequencies of external compressive stresses. The work complements existing works that only consider the static cases. The work can shed light in future high frequency piezoelectronic devices exploration., Comment: 13 pages, 11 figures

Piezoelectric energy harvester scavenges mechanical vibrations and generates electricity. Researchers have strived to optimize the electromechanical structures and to design necessary external power management circuits, aiming to deliver high power and rectified outputs ready for serving as batteries. Complex deformation of the mechanical structure results in charges with opposite polarities appearing on same surface, leading to current loss in the attached metal electrode. External power management circuits such as rectifiers comprise diodes that consume power and have undesirable forward bias. To address the above issues, we devise a novel integrated piezoelectric energy harvesting device that is structured by stacking a layer of quantum dots (QDs) and a layer of piezoelectric material. We find that the QD can rectify electrical charges generated from the piezoelectric material because of its adaptable conductance to the electrochemical potentials of both sides of the QDs layer, so that electrical current causing energy loss on the same surface of the piezoelectric material can be minimized. The QDs layer has the potential to replace external rectification circuits providing a much more compact and less power-consumption solution., Comment: 10 pages, 7 figures

The performance of thermoelectric energy harvesters can be improved by nanostructures that exploit inelastic transport processes. One prototype is the three-terminal hopping thermoelectric device where electron hopping between quantum-dots are driven by hot phonons. Such three-terminal hopping thermoelectric devices have potential in achieving high efficiency or power via inelastic transport and without relying on heavy-elements or toxic compounds. We show in this work how output power of the device can be optimized via tuning the number and energy configuration of the quantum-dots embedded in parallel nanowires. We find that the staircase energy configuration with constant energy-step can improve the power factor over a serial connection of a single pair of quantum-dots. Moreover, for a fixed energy-step, there is an optimal length for the nanowire. Similarly for a fixed number of quantum-dots there is an optimal energy-step for the output power. Our results are important for future developments of high-performance nanostructured thermoelectric devices., Comment: 9 pages, 5 figures

Thermoelectric energy harvesters can have a much higher conversion efficiency by implementing quantum dots/wells between the high temperature region and the low temperature region. However they still suffer a limitation of the maximum output power, represented by the maximum $\Delta E$ (maximum energy gap of two quantum dots/wells layers). In this work, we use the piezoelectric material in the high temperature region, which has conceptually addressed the problem of the maximum power limitation. Full analysis of device physics including comparison with the existing technology and quantum simulation has been conducted to validate this concept. Results show that with the new concept, the maximum output power has been increased by at least an order of magnitude with the same power input and identical device dimensions., Comment: 8 pages, 6 figures

We address the problem of quantum transport in a nanometre sized two-terminal ZnO device subject to an external strain. The two junctions formed between the electrodes and the ZnO are generally taken as Ohmic and Schottky type, respectively. Unlike the conventional treatment to the piezopotential, we treat it as a potential barrier which is only induced at the interfaces. By calculating the transmission coefficient of a Fermi-energized electron that flows from one end to the other, it is found that the piezopotential has the effect of modulating the voltage threshold of the current flowing. The calculations are based on the quantum scattering theory. The work is believed to pave the way for investigating the quantum piezotronics., Comment: 7 pages