Yit-Tsong Chen

High Performance and Bendable Few-Layered InSe Photodetectors with Broad Spectral Response

Two-dimensional crystals with a wealth of exotic dimensional-dependent properties are promising candidates for next-generation ultrathin and flexible optoelectronic devices. For the first time, we demonstrate that few-layered InSe photodetectors, fabricated on both a rigid SiO2/Si substrate and a flexible polyethylene terephthalate (PET) film, are capable of conducting broadband photodetection from the visible to near-infrared region (450-785 nm) with high photoresponsivities of up to 12.3 AW(-1) at 450 nm (on SiO2/Si) and 3.9 AW(-1) at 633 nm (on PET). These photoresponsivities are superior to those of other recently reported two-dimensional (2D) crystal-based (graphene, MoS2, GaS, and GaSe) photodetectors. The InSe devices fabricated on rigid SiO2/Si substrates possess a response time of ∼50 ms and exhibit long-term stability in photoswitching. These InSe devices can also operate on a flexible substrate with or without bending and reveal comparable performance to those devices on SiO2/Si. With these excellent optoelectronic merits, we envision that the nanoscale InSe layers will not only find applications in flexible optoelectronics but also act as an active component to configure versatile 2D heterostructure devices.

The photoluminescence from hydrogen-related species in composites of SiO2 nanoparticles

Measurements of photoluminescence (PL) from composites of silica nanoparticles (the primary particle size 7 and 15 nm) as a function of heat treatment temperature show that the PL results from hydrogen-related species and thermally produced structural defects. The PL was induced by an ArF or Nd:YAG (yttrium–aluminum–garnet) laser (λexc=193 or 266 nm). The green PL exhibits a progression with spacings of about Δν=630 cm−1 assigned to the bending vibration of ≡Si–H on the surface of particles. The spacings increase up to Δν=1200 cm−1 when ≡Si–H and nonbridging oxygen (≡Si–O•) form interfacial water species.

Label-free detection of protein-protein interactions using a calmodulin-modified nanowire transistor

In this study, we describe a highly sensitive and reusable silicon nanowire field-effect transistor for the detection of protein-protein interactions. This reusable device was made possible by the reversible association of glutathione S-transferase-tagged calmodulin with a glutathione modified transistor. The calmodulin-modified transistor exhibited selective electrical responses to Ca2+ (≥1 μM) and purified cardiac troponin I (∼7 nM); the change in conductivity displayed a linear dependence on the concentration of troponin I in a range from 10 nM to 1 μM. These results are consistent with the previously reported concentration range in which the dissociation constant for the troponin I-calmodulin complex was determined. The minimum concentration of Ca2+ required to activate calmodulin was determined to be 1 μM. We have also successfully demonstrated that the N-type Ca2+ channels, expressed by cultured 293T cells, can be recognized specifically by the calmodulin-modified nanowire transistor. This sensitive nanowire transistor can serve as a high-throughput biosensor and can also substitute for immunoprecipitation methods used in the identification of interacting proteins.

High-Quality Graphene p−n Junctions via Resist-free Fabrication and Solution-Based Noncovalent Functionalization

An essential issue in graphene nanoelectronics is to engineer the carrier type and density and still preserve the unique band structure of graphene. We report the realization of high-quality graphene p-n junctions by noncovalent chemical functionalization. A generic scheme for the graphene p-n junction fabrication is established by combining the resist-free approach and spatially selective chemical modification process. The effectiveness of the chemical functionalization is systematically confirmed by surface topography and potential measurements, spatially resolved Raman spectroscopic imaging, and transport/magnetotransport measurements. The transport characteristics of graphene p-n junctions are presented with observations of high carrier mobilities, Fermi energy difference, and distinct quantum Hall plateaus. The chemical functionalization of graphene p-n junctions demonstrated in this study is believed to be a feasible scheme for modulating the doping level in graphene for future graphene-based nanoelectronics.

Intrinsic Electron Mobility Exceeding 103 cm2/(V s) in Multilayer InSe FETs

Graphene-like two-dimensional (2D) materials not only are interesting for their exotic electronic structure and fundamental electronic transport or optical properties but also hold promises for device miniaturization down to atomic thickness. As one material belonging to this category, InSe, a III-VI semiconductor, not only is a promising candidate for optoelectronic devices but also has potential for ultrathin field effect transistor (FET) with high mobility transport. In this work, various substrates such as PMMA, bare silicon oxide, passivated silicon oxide, and silicon nitride were used to fabricate multilayer InSe FET devices. Through back gating and Hall measurement in four-probe configuration, the devices field effect mobility and intrinsic Hall mobility were extracted at various temperatures to study the materials intrinsic transport behavior and the effect of dielectric substrate. The samples field effect and Hall mobilities over the range of 20-300 K fall in the range of 0.1-2.0 × 10(3) cm(2)/(V s), which are comparable or better than the state of the art FETs made of widely studied 2D transition metal dichalcogenides.

An Ultrasensitive Nanowire-Transistor Biosensor for Detecting Dopamine Release from Living PC12 Cells under Hypoxic Stimulation

Dopamine (DA) is an important neurotransmitter that is involved in neuronal signal transduction and several critical illnesses. However, the concentration of DA is extremely low in patients and is difficult to detect using existing electrochemical biosensors with detection limits typically around nanomolar levels (∼10(-9) M). Here, we developed a nanoelectronic device as a biosensor for ultrasensitive and selective DA detection by modifying DNA-aptamers on a multiple-parallel-connected (MPC) silicon nanowire field-effect transistor (referred to as MPC aptamer/SiNW-FET). Compared with conventional electrochemical methods, the MPC aptamer/SiNW-FET has been demonstrated to improve the limit of DA detection to <10(-11) M and to possess a detection specificity that is able to distinguish DA from other chemical analogues, such as ascorbic acid, catechol, phenethylamine, tyrosine, epinephrine, and norepinephrine. This MPC aptamer/SiNW-FET was also applied to monitor DA release under hypoxic stimulation from living PC12 cells. The real-time recording of the exocytotic DA induced by hypoxia reveals that the increase in intracellular Ca(2+) that is required to trigger DA secretion is dominated by an extracellular Ca(2+) influx, rather than the release of intracellular Ca(2+) stores.

Intrinsic electron mobility exceeding 1000 cm

Graphene-like two-dimensional (2D) materials not only are interesting for their exotic electronic structure and fundamental electronic transport or optical properties but also hold promises for device miniaturization down to atomic thickness. As one material belonging to this category, InSe, a III-VI semiconductor, not only is a promising candidate for optoelectronic devices but also has potential for ultrathin field effect transistor (FET) with high mobility transport. In this work, various substrates such as PMMA, bare silicon oxide, passivated silicon oxide, and silicon nitride were used to fabricate multilayer InSe FET devices. Through back gating and Hall measurement in four-probe configuration, the devices field effect mobility and intrinsic Hall mobility were extracted at various temperatures to study the materials intrinsic transport behavior and the effect of dielectric substrate. The samples field effect and Hall mobilities over the range of 20-300 K fall in the range of 0.1-2.0 × 10(3) cm(2)/(V s), which are comparable or better than the state of the art FETs made of widely studied 2D transition metal dichalcogenides.

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We have observed the IR spectra of the melamine molecule and its deuteriated counterpart in the gas phase at ca. 150°C and in a solid argon-matrix at 10 K. The assignment of the vibrations of melamine has been facilitated by the calculated thirty nine normal modes using several abinitio and density functional methods. By scaling the calculated vibrational frequencies, the theoretical computations have been demonstrated to be in good agreement with the experimental observations. The optimized equilibrium structure of melamine has been shown to be a planar but distorted-hexagonal triazine ring with three pyramidal amino groups, which result in different conformers. This has been supported by the comparison between the observed and the calculated spectra for non-planar conformers 1 and 2 vs. the planar D3h structure 3. In view of the small energy differences between the calculated conformers 1 and 2 and the ‘transition state’ 3 (corresponding to a third-order saddle point on the potential-energy hypersurface), the melamine molecule has a flat potential-energy hypersurface near the equilibrium structures and the conformers can rapidly rearrange.

/Vs in multilayer InSe FETs

The vibronic spectra of ethylene have been studied using ab initio molecular orbital methods. Geometries of the singlet π–π*, π–3s, and π–3p excited electronic states have been optimized at the CIS and CASSCF levels of theory with the 6‐311(2+)G* basis set. Vertical and adiabatic excitation energies, calculated by the multireference configuration interaction (MRCI) and equation‐of‐motion coupled cluster (EOM‐CCSD) methods are in quantitative agreement with experiment. Vibrational frequencies and normal coordinates for the ground and excited states are used for the calculations of vibrational overlap integrals and Franck–Condon factors, taking into account distortion, displacement, and normal mode mixing (up to four modes). Major features of the observed absorption spectrum of ethylene have been interpreted on the basis of the computed Franck–Condon factors. The role of each electronic state in the spectra has been clarified; the π–3s transition corresponds to the distinct intensive peaks in the 57 000–61 ...

IR SPECTROSCOPY AND THEORETICAL VIBRATIONAL CALCULATION OF THE MELAMINE MOLECULE

Ab initio calculations of geometry and vibrational frequencies of the first singlet excited 1A2(1A″) state of acetone corresponding to the n-π* electronic transition have been carried out at the CASSCF/6-311G** level. The major geometry changes in this state as compared to the ground state involve CO out-of-plane wagging, CO stretch and torsion of the methyl groups, and the molecular symmetry changes from C2v to Cs. The most pronounced frequency changes in the 1A″ state are the decrease of the CO stretch frequency v3 by almost 500 cm−1 and the increase of the CH3 torsion frequency v12 from 22 to 170 cm−1. The optimized geometries and normal modes are used to compute the normal mode displacements which are applied for calculations of Franck–Condon factors. Transition matrix elements over the one-electron electric field operator at various atomic centers calculated at the state-average CASSCF/6-311+G** level are used to compute vibronic couplings between the ground 1A1, 1A2, and Rydberg 1B2(n-3s), 2 1A1(n-3...