X. S. Chen

Ferromagnetism in Semihydrogenated Graphene Sheet

Single layer of graphite (graphene) was predicted and later experimentally confirmed to undergo metal-semiconductor transition when fully hydrogenated (graphane). Using density functional theory we show that when half of the hydrogen in this graphane sheet is removed, the resulting semihydrogenated graphene (which we refer to as graphone) becomes a ferromagnetic semiconductor with a small indirect gap. Half-hydrogenation breaks the delocalized pi bonding network of graphene, leaving the electrons in the unhydrogenated carbon atoms localized and unpaired. The magnetic moments at these sites couple ferromagnetically with an estimated Curie temperature between 278 and 417 K, giving rise to an infinite magnetic sheet with structural integrity and magnetic homogeneity. This is very different from the widely studied finite graphene nanostrucures such as one-dimensional nanoribbons and two-dimensional nanoholes, where zigzag edges are necessary for magnetism. From graphene to graphane and to graphone, the system evolves from metallic to semiconducting and from nonmagnetic to magnetic. Hydrogenation provides a novel way to tune the properties with unprecedented potentials for applications.

Electric field enhanced hydrogen storage on polarizable materials substrates

Using density functional theory we show that an applied electric field substantially improves the hydrogen storage properties of a BN sheet by polarizing the hydrogen molecules as well as the substrate. The adsorption energy of a single H2 molecule in the presence of an electric field of 0.05 a.u. is 0.48 eV compared to 0.07 eV in its absence. When one layer of H2 molecules is adsorbed, the binding energy per H2 molecule increases from 0.03 eV in the field-free case to 0.14 eV/H2 in the presence of an electric field of 0.045 a.u. The corresponding gravimetric density of 7.5 wt % is consistent with the 6 wt % system target set by DOE for 2010. Once the applied electric field is removed, the stored H2 molecules can be easily released, thus making the storage reversible.Using density functional theory, we show that an applied electric field can substantially improve the hydrogen storage properties of polarizable substrates. This new concept is demonstrated by adsorbing a layer of hydrogen molecules on a number of nanomaterials. When one layer of H2 molecules is adsorbed on a BN sheet, the binding energy per H2 molecule increases from 0.03 eV/H2 in the field-free case to 0.14 eV/H2 in the presence of an electric field of 0.045 a.u. The corresponding gravimetric density of 7.5 wt% is consistent with the 6 wt% system target set by Department of Energy for 2010. The strength of the electric field can be reduced if the substrate is more polarizable. For example, a hydrogen adsorption energy of 0.14 eV/H2 can be achieved by applying an electric field of 0.03 a.u. on an AlN substrate, 0.006 a.u. on a silsesquioxane molecule, and 0.007 a.u. on a silsesquioxane sheet. Thus, application of an electric field to a polarizable substrate provides a novel way to store hydrogen; once the applied electric field is removed, the stored H2 molecules can be easily released, thus making storage reversible with fast kinetics. In addition, we show that materials with rich low-coordinated nonmetal anions are highly polarizable and can serve as a guide in the design of new hydrogen storage materials.

Two-dimensional transient simulations of drain lag and current collapse in GaN-based high-electron-mobility transistors

The intrinsic mechanisms of drain lag and current collapse in GaN-based high-electron-mobility transistors are studied by using two-dimensional numerical simulations. Simulated drain lag characteristics are in good agreement with reported experimental data. The dynamic pictures of trapping of hot electrons under drain-pulse voltages are discussed in detail. Hot-electron buffer-trapping effect plays an instrumental role in the current collapse mechanism. Polarization-induced interface charges have significant effect on the hot-electron buffer trapping and the current collapse can be weakened by increasing the interface charges. The trapped charges can accumulate at the drain-side gate edge, where the electric field significantly changes and gate-to-drain-voltage-dependent strain is induced, causing a notable current collapse. The simulation results show that the drain voltage range, beyond 5 V, is already in the field of the well-developed hot electron regime. The hot electrons can occupy a great number of...

Simulation and optimization of GaN-based metal-oxide-semiconductor high-electron-mobility-transistor using field-dependent drift velocity model

Undoped GaN-based metal-oxide-semiconductor high-electron-mobility-transistors (MOS-HEMTs) with atomic-layer-deposited Al2O3 gate dielectrics are fabricated with gate lengths from 1 μm up to 40 μm. With a two-dimensional numerical simulator, we report simulation results of the GaN-based MOS-HEMTs using field-dependent drift velocity model. A developed model, taking into account polarization-induced charges and defect-induced traps at all of the interfaces and process-related trap levels of bulk traps measured from experiments, is built. The simulated output characteristics are in good agreement with reported experimental data. The effect of the high field at the drain-side gate edge and bulk trap density of GaN on the output performance is discussed in detail for the device optimization. AlGaN/GaN/AlN quantum-well (QW) MOS-HEMTs have been proposed and demonstrated based on numerical simulations. The simulation results also link the current collapse with electrons spreading into the bulk, and confirm that ...

Effects from A-site substitution on morphotropic phase boundary and phonon modes of (Pb1–1.5xLax)(Zr0.42Sn0.40Ti0.18)O3 ceramics by temperature dependent Raman spectroscopy

The complex perovskite ferroelectric/antiferroelectric of (Pb 1−1.5 x La x )(Zr0.42Sn0.40Ti0.18)O3 (PLZST) ceramics have been investigated by Raman scattering spectra from 77 to 480 K. It was found that phase transition occurs between La composition of 2.6% and 2.8% for PLZST ceramics. Softing of A1(TO1) mode and dramatic changes of relative strength from E(TO2) mode are observed at morphotropic phase boundary (MPB). Moreover, it was found that MPB characteristic shows a wider and lower trend of temperature region with increasing La composition. This could be ascribed to the diminishment of the energy barrier and increment of A-cation entropy.

The role of ultrathin AlN barrier in the reduction in the hot electron and self-heating effects for GaN-based double-heterojunction high electron mobility transistors

We propose an AlN/GaN/InGaN/GaN double-heterojunction high electron mobility transistor (DH-HEMT) structure with a 4 nm thin AlN barrier layer. The performance of the DH-HEMT device is investigated by using two-dimensional numerical simulation. The conduction band profile is obtained by using the Poisson’s equation and Fermi–Dirac statistics in combination with the polarization charges. Due to large conduction-band offset of the AlN/GaN interface and strong polarization of AlN, the minor channel at GaN/InGaN interface can be eliminated. Further, the hot electron and self-heating effects on the transport properties of this DH-HEMT are investigated by using hydrodynamic model. In comparison with the AlGaN barrier DH-HEMT and conventional HEMT, this kind of DH-HEMT can effectively reduce the hot electron effect under high voltage. The reason is that the maximum field strength is far below the critical value for the existence of the hot electron effect in the AlGaN barrier DH-HEMTs and conventional HEMTs with...

Abnormal electronic transition variations of lanthanum-modified lead zironate stannate titanate ceramics near morphotropic phase boundary: A spectroscopic evidence

The structure-related optical response of (Pb{sub 1-1.5x}La{sub x})(Zr{sub 0.42}Sn{sub 0.40}Ti{sub 0.18})O{sub 3} (100x/42/40/18) ceramics with different compositions has been investigated. Based on x-ray diffraction, the phase transition from rhombohedral to tetragonal structure is revealed between compositions of x = 2.6% and 2.8% near morphotropic phase boundary (MPB). Correspondingly, abnormal spectral response in the photon energy from 1.4 to 6.1 eV is observed near MPB. Furthermore, the blue shift of the two critical points related parameters, which is obtained from fitting the reflectance spectra, indicates that the variation of electronic band structure near MPB is responsible for the anomalous behavior.

Interferon regulatory factor 4 regulates thymocyte differentiation by repressing Runx3 expression

The transcription factor interferon regulatory factor 4 (IRF4) was originally found to be preferentially expressed in lymphoid cells and to be required for the function, differentiation, and homeostasis of both mature T and B lymphocytes. Recent studies have indicated that IRF4 is also involved in early B‐cell development. However, the role of IRF4 in intrathymic T‐cell development remains unknown. In this study, we show that IRF4 is upregulated in TCR‐signaled thymocytes and is predominantly expressed in CD4 single‐positive (SP), but not in CD8 SP, cells. T‐cell‐specific overexpression of IRF4 impaired the generation and maturation of CD8 SP thymocytes. Further analysis revealed that IRF4 selectively bound to the distal promoter region of Runx3 and repressed its transcription, probably through the deacetylation of histones H3 and H4 in intermediate CD4+CD8low cells and CD4 SP thymocytes. Similar to the effect of Runx3 deficiency, transgenic expression of IRF4 led not only to an aberrantly high expression of CD4 surface molecules on intermediate CD4+CD8low cells and CD8 SP thymocytes, but also impaired CD8+ T‐cell function. Taken together, our data suggest that IRF4 plays an important role in the regulation of Runx3 expression and CD4+/CD8+ thymocyte differentiation.

Rapid microwave phase detection based on a solid state spintronic device

A technique for rapidly detecting microwave magnitude and phase has been developed using a spintronic device as a microwave sensor, which allows a lock-in amplifier to perform real-time microwave measurement. To demonstrate the feasibility and reliability of the proposed approach, the resonance including the amplitude and phase in a complementary electric inductive-capacitive resonator has been characterized. The results are in agreement with measurement preformed by a vector network. This sensor approach is not limited for use only with spintronic devices, but can also be used with semiconductor devices and hence offers a useful alternative to existing microwave imaging and characterization technologies.A technique for rapidly detecting microwave phase has been developed which uses a spintronic device that can directly rectify microwave fields into a dc voltage signal. Use of a voltage-controlled phase shifter enables the development of a spintronic device that can simultaneously ”read” the magnitude and phase of incident continuous-wave (CW) microwaves when combined with a lock-in amplifier. As an example of many possible practical applications of this device, the resonance phase in a complementary electric inductive-capacitive (CELC) resonator has been characterized using a spintronic sensor based on a magnetic tunnel junction (MTJ). This sensor device is not limited for use only with spintronic devices such as MTJs, but can also be used with semiconductor devices such as microwave detectors, and hence offers a useful alternative to existing microwave imaging and characterization technologies.

Different temperature dependence of excitonic and defect-related photoluminescence spectra in ZnS nanobelts and nanowires

In this paper, both excitonic and defect-related information of ZnS nanobelts and nanowires have been investigated by a temperature-dependent photoluminescence (PL) spectrum. PL spectra of ZnS nanobelts and nanowires differ significantly in the ultraviolet (UV) and visible emission regions. In UV emission regions, due to high-quality crystals, free exciton B (FXB), free exciton A (FXA), FXA-one longitudinal optical (LO) phonon replica are observed in ZnS nanobelts, as well as free-to-bound (e, A) with its one LO phonon replica, while neutral-donor bound exciton (Do, X) and free-to-bound (e, A) are observed in ZnS nanowires at 10 K. The peak and relative intensity of the FX and (Do, X) versus temperature follow well with conventional empirical relations. In the visible emission regions, weak donor–acceptor pair (DAP) and self-activated (SA) emission from ZnS nanowires are commonly observed, but the Y band emission is only observed at 10 K in ZnS nanobelts. The Y band emission disappears at some temperature lower than 50 K. The peak position and full width at half maximum of DAP and SA emission bands display different temperature dependences. Detailed study on temperature-dependent PL spectra of ZnS nanobelts and nanowires provides crucial information on the nature of the electronic states and recombination mechanisms in these nanostructures.