A gate-free monolayer WSe2 pn diode.

Interest in bringing p- and n-type monolayer semiconducting transition metal dichalcogenides (TMD) into contact to form rectifying pn diode has thrived since it is crucial to control the electrical properties in two-dimensional (2D) electronic and optoelectronic devices. Usually this involves vertically stacking different TMDs with pn heterojunction or, laterally manipulating carrier density by gate biasing. Here, by utilizing a locally reversed ferroelectric polarization, we laterally manipulate the carrier density and created a WSe2 pn homojunction on the supporting ferroelectric BiFeO3 substrate. This non-volatile WSe2 pn homojunction is demonstrated with optical and scanning probe methods and scanning photoelectron micro-spectroscopy. A homo-interface is a direct manifestation of our WSe2 pn diode, which can be quantitatively understood as a clear rectifying behavior. The non-volatile confinement of carriers and associated gate-free pn homojunction can be an addition to the 2D electron-photon toolbox and pave the way to develop laterally 2D electronics and photonics.

Elucidating Quantum Confinement in Graphene Oxide Dots Based On Excitation-Wavelength-Independent Photoluminescence.

Investigating quantum confinement in graphene under ambient conditions remains a challenge. In this study, we present graphene oxide quantum dots (GOQDs) that show excitation-wavelength-independent photoluminescence. The luminescence color varies from orange-red to blue as the GOQD size is reduced from 8 to 1 nm. The photoluminescence of each GOQD specimen is associated with electron transitions from the antibonding π (π*) to oxygen nonbonding (n-state) orbitals. The observed quantum confinement is ascribed to a size change in the sp(2) domains, which leads to a change in the π*-π gap; the n-state levels remain unaffected by the size change. The electronic properties and mechanisms involved in quantum-confined photoluminescence can serve as the foundation for the application of oxygenated graphene in electronics, photonics, and biology.

Stress-strain analysis for evaluating the effect of the orientation of dentin tubules on their mechanical properties and deformation behavior.

A model whose porosity does not vary with compression depth is developed for evaluating the mechanical properties of dentin tubules with various orientation angles from micro-pillar nanocompression tests. Experimental results for a range of loading rates indicate that the yielding parameters and the elastic modulus are little affected by the creep behavior. For a given compression depth, the hardness, elastic modulus, and yielding strength decrease with increasing orientation angle of dentin. The mechanical properties obtained using the proposed model are consistent with the reported data, and are actually more precise since they consider the orientation angle. The proposed testing method can be applied to materials that yield a negative value of the elastic modulus due to creep behavior.

Synthesis and bulk acoustic wave properties on the dual mode frequency shift of solidly mounted resonators.

This study focused on the fabrication and the theoretical analysis of solidly mounted resonators (SMR) concerning dual-mode frequency responses and their frequency shift of bulk acoustic wave (BAW) resonance. For this device fabrication, RF/DC magnetron sputtering and photolithography were employed to constitute the required multilayer structure. For the theoretical analysis, the dualmode frequency shift was characterized by the Sauerbrey's formula, and a modified formula was carried out following the trend for the large frequency shift. In the fabrication of the SMR device, Mo/SiO2 was chosen to construct the Bragg reflector as the high/low acoustic impedance materials, respectively, and aluminum nitride (AlN) was used as a piezoelectric layer. To investigate the characteristics of BAW on the dual-mode frequency shift, the c-axis tilted angle of AlN was altered as well as the various mass loading on the SMR. Based on the experimental results, the dual-resonance frequencies showed a nonlinear decreasing trend with a linear increase of the mass loading. Therefore, a modified formula was carried out. Furthermore, the ratio of the longitudinal-resonant frequency to the shear-resonant frequency remained at a range around 1.76 despite the various c-axis tilted angles of AlN and gradual mass loading on the SMR. The electromechanical coupling coefficient, k2(eff), of the shear resonance rose with the increase of the c-axis tilted angle of AlN.

Influence of surface roughness of Bragg reflectors on resonance characteristics of solidly-mounted resonators.

The solidly mounted resonator (SMR) is fabricated using planar processes from a piezoelectric layer sandwiched between two electrodes upon Bragg reflectors, which then are attached to a substrate. To transform the effective acoustic impedance of the substrate to a near zero value, the Bragg reflectors are composed of alternating high and low acoustic impedance layers of quarter-wavelength thickness. This paper presents the influence of Bragg reflector surface roughness on the resonance characteristics of an SMR. Originally, an A1N/A1 multilayer is used as the Bragg reflector. The poor surface roughness of this Bragg reflector results in a poor SMR frequency response. To improve the surface roughness of Bragg reflectors, a molybdenum (Mo)/titanium (Ti) multilayer with a similar coefficient of thermal expansion is adopted. By controlling deposition parameters, the surface roughness of the Bragg reflector is improved, and better resonance characteristics of SMR are obtained.

Proton-exchanged 36 degrees Y-X LiTaO3 waveguides for surface acoustic wave.

A nontoxic proton source, octanoic acid, was adopted to fabricate proton-exchanged (PE) waveguides in 36 degrees Y-X lithium tantalate (LiTaO3) substrates. The PE ability of octanoic acid on LiTaO3, the penetration depth, was investigated by secondary-ion mass spectrometry (SIMS). The penetration depth of hydrogen ion exhibited an obviously step-like profile, which will be excellent for waveguide application. The relationship between waveguide depth (d) and exchanging time (t) was represented by d = 0.0653 X square root of t at T = 200 degrees C. To deserve to be mentioned, the octanoic acid has a slight dissociation coefficient and low activation energy, thus the accurate waveguide depth control can be obtained. For the application of acoustic wave guided acousto-optic devices, the leaky surface acoustic wave (LSAW) properties of PE 36 degrees Y-X LiTaO3 waveguides were investigated. The phase velocity slightly decreased with the increase of kd, where k was wavenumber. An indispensable parameter of acoustic wave device, the temperature coefficient of frequency (TCF), calculated from the frequency change of the output of LSAW delay line showed an increase with increased kd.

Surface acoustic wave properties of proton-exchanged LiNbO3 waveguides with SiO2 film.

Surface acoustic wave (SAW) properties of proton-exchanged (PE) z-cut lithium niobate (LiNbO3) waveguides with silicon dioxide (SiO2) film layers were investigated using octanoic acid. The distribution of hydrogen measured by secondary ion mass spectrometry (SIMS) showed a step-like profile, which was assumed to be equal to the waveguide depth (d). The SiO2 film was deposited on z-cut LiNbO3 waveguide by radio frequency (rf) magnetron sputtering. We investigated the important parameters for the design of SAW devices such as phase velocity (Vp), insertion loss (IL) and temperature coefficient of frequency (TCF) by a network analyzer using thin-film aluminum interdigital transducer electrodes on the upper SiO2 film surface. The experimental results showed that the Vp of SAW decreased slightly with the increase of h/lambda, where h was the thickness of SiO2 films and lambda was the wavelength. The IL of SAW increased with increased h/lambda. The TCF of SAW calculated from the frequency change of the output of SAW delay line showed an evident decrease with the increase of h/lambda. The TCF for PE z-cut LiNbO3 was measured to be about -54.72 ppm/degreees C at h/lambda = 0.08. It revealed that the SiO2 films could compensate and improve the temperature stability as compared with the TCF of SAW on PE samples without SiO2 film.