Transient Absorption Measurements on Anisotropic Monolayer ReS2.

Anisotropic optical and transport properties of monolayer ReS2 fabricated by mechanical exfoliation are reported. Transient absorption measurements with different polarization configurations and sample orientations reveal that the absorption coefficient and transient absorption are both anisotropic, with maximal and minimal values occurring when the light polarization is parallel and perpendicular to the Re atomic chains, respectively. The maximal values are about a factor of 2.5 of the minimal values. By resolving the spatiotemporal dynamics of excitons, it is found that the diffusion coefficient of excitons moving along Re atomic chains is about 16 cm(2) s(-1) at room temperature, which is about a factor of three larger than those moving perpendicular to that direction. An exciton lifetime of 40 ps is also extracted. These findings establish monolayer ReS2 as an anisotropic 2D transition metal dichalcogenide.

The graphene-gold interface and its implications for nanoelectronics.

We combine optical microspectroscopy and electronic measurements to study how gold deposition affects the physical properties of graphene. We find that the electronic structure, the electron-phonon coupling, and the doping level in gold-plated graphene are largely preserved. The transfer lengths for electrons and holes at the graphene-gold contact have values as high as 1.6 µm. However, the interfacial coupling of graphene and gold causes local temperature drops of up to 500 K in operating electronic devices.

Controllable p-n junction formation in monolayer graphene using electrostatic substrate engineering.

We investigate electric transport in graphene on SiO2 in the high field limit and report on the formation of p-n junctions. Previously, doping of graphene has been achieved by using multiple electrostatic gates, or charge transfer from adsorbants. Here we demonstrate a novel approach to create p-n junctions by changing the local electrostatic potential in the vicinity of one of the contacts without the use of extra gates. The approach is based on the electronic modification not of the graphene but of the substrate and produces a well-behaved, sharp junction whose position and height can be controlled.

Utilization of a buffered dielectric to achieve high field-effect carrier mobility in graphene transistors.

We utilize an organic polymer buffer layer between graphene and conventional gate dielectrics in top-gated graphene transistors. Unlike other insulators, this dielectric stack does not significantly degrade carrier mobility, allowing for high field-effect mobilities to be retained in top-gate operation. This is demonstrated in both two-point and four-point analysis and in the high-frequency operation of a graphene transistor. Temperature dependence of the carrier mobility suggests that phonons are the dominant scatterers in these devices.

A Randomized Controlled Trial on the Role of Enthusiasm About Exergames: Players' Perceptions of Exercise.

Objective: Exergames are popular technology applications that encourage individuals to engage in exercise and create positive moods for players. However, little is known as to whether playing exergames makes players perceive to be more energetic and relaxed and whether enthusiasm about doing exercise moderates such perceptions. To answer these questions, we use the Flow Theory and the Self-Determination Theory to develop the hypotheses. Methods: We conducted a randomized controlled trial, which randomly assigned 337 participants to an intervention group and a control group. We asked the participants in the intervention group to play exergames for 2 weeks. We measured enthusiasm about doing exercise by asking the participants to evaluate themselves as having enthusiasm on doing exercise or not. We measured participants' perceptions of happiness, perceived energy (the perception of sufficient physical and mental resources), and relaxation before and after the 2-week exergame playing, generating scores to represent their changes. Results: We found that playing exergames induces positive changes in happiness, perceived energy, and relaxation. Such changes were significant for participants who are enthusiastic about doing exercise, but not for those who are unenthusiastic about doing exercise. Conclusion: This study was the first using the Flow Theory and the Self-Determination Theory to examine the impact of playing exergames on players' perceptions and to identify the moderator role of enthusiasm about doing exercise. These positive impacts of exergames can be used in rehabilitation settings in encouraging positive attitudes and behaviors toward exercise.

Impact of Playing Exergames on Mood States: A Randomized Controlled Trial.

To examine how playing exergames impacts the mood states of university students and staff, and whether such an impact depends on gender and players' previous exercise time. This study was designed as a randomized controlled trial. It enrolled 337 participants and randomly assigned them to an intervention group (n = 168) or a control group (n = 167). A 2-week exergame program was designed for the participants in the intervention group. They were required to play exergames for 30 consecutive minutes each week for 2 weeks and respond to the items measuring vigor, happiness, and perceived stress. All measures were administered before and after the study. Repeated measures analysis of variances were conducted. Playing exergames enhanced vigor and happiness for participants in the intervention group. This group exhibited more positive change in vigor and happiness than the control group. This effect of playing exergames was not moderated by gender, age, occupation (student or staff), or previous exercise time. Playing exergames may induce positive mood states among university students and staff.

Probing charge transfer excitons in a MoSe2-WS2 van der Waals heterostructure.

We show that the van der Waals heterostructure formed by MoSe2 and WS2 provides a unique system with near degenerate interlayer and intralayer excitonic states. Photoluminescence measurements indicate that the charge transfer exciton states are approximately 50 meV below the MoSe2 exciton states, with a significant spectral overlap. The transient absorption of a femtosecond pulse was used to study the dynamics of the charge transfer excitons at room temperature. We found a lifetime of approximately 80 ps for the charge transfer excitons. A diffusion coefficient of about 14 cm(2) s(-1) was deduced, which is comparable to individual excitons in transition metal dichalcogenides.

Tightly Bound Trions in Transition Metal Dichalcogenide Heterostructures.

We report the observation of trions at room temperature in a van der Waals heterostructure composed of MoSe2 and WS2 monolayers. These trions are formed by excitons excited in the WS2 layer and electrons transferred from the MoSe2 layer. Recombination of trions results in a peak in the photoluminescence spectra, which is absent in monolayer WS2 that is not in contact with MoSe2. The trion origin of this peak is further confirmed by the linear dependence of the peak position on excitation intensity. We deduced a zero-density trion binding energy of 62 meV. The trion formation facilitates electrical control of exciton transport in transition metal dichalcogenide heterostructures, which can be utilized in various optoelectronic applications.

Exceptional and Anisotropic Transport Properties of Photocarriers in Black Phosphorus.

One key challenge in developing postsilicon electronic technology is to find ultrathin channel materials with high charge mobilities and sizable energy band gaps. Graphene can offer extremely high charge mobilities; however, the lack of a band gap presents a significant barrier. Transition metal dichalcogenides possess sizable and thickness-tunable band gaps; however, their charge mobilities are relatively low. Here we show that black phosphorus has room-temperature charge mobilities on the order of 10(4) cm(2) V(-1) s(-1), which are about 1 order of magnitude larger than silicon. We also demonstrate strong anisotropic transport in black phosphorus, where the mobilities along the armchair direction are about 1 order of magnitude larger than in the zigzag direction. A photocarrier lifetime as long as 100 ps is also determined. These results illustrate that black phosphorus is a promising candidate for future electronic and optoelectronic applications.

Ultrafast charge separation and indirect exciton formation in a MoS2-MoSe2 van der Waals heterostructure.

We observe subpicosecond charge separation and formation of indirect excitons a van der Waals heterostructure formed by molybdenum disulfide and molybdenum diselenide monolayers. The sample is fabricated by manually stacking monolayer MoS2 and MoSe2 flakes prepared by mechanical exfoliation. Photoluminescence measurements confirm the formation of an effective heterojunction. In the transient absorption measurements, an ultrafast laser pulse resonantly injects excitons in the MoSe2 layer of the heterostructure. Differential reflection of a probe pulse tuned to the MoS2 exciton resonance is immediately observed following the pump excitation. This proves ultrafast transfer of electrons from MoSe2 to MoS2 layers, despite the strong Coulomb attraction from the holes in the resonantly excited excitons. Conversely, when excitons are selectively injected in MoS2, holes transfer to MoSe2 on an ultrafast time scale, too, as observed by measuring the differential reflection of a probe tuned to the MoSe2 resonance. The ultrafast charge transfer process is followed by the formation of spatially indirect excitons with electrons and holes residing in different layers. The lifetime of these indirect excitons are found to be longer than that of the direct excitons in individual MoS2 and MoSe2 monolayers.

Electron transfer and coupling in graphene-tungsten disulfide van der Waals heterostructures.

The newly discovered two-dimensional materials can be used to form atomically thin and sharp van der Waals heterostructures with nearly perfect interface qualities, which can transform the science and technology of semiconductor heterostructures. Owing to the weak van der Waals interlayer coupling, the electronic states of participating materials remain largely unchanged. Hence, emergent properties of these structures rely on two key elements: electron transfer across the interface and interlayer coupling. Here we show, using graphene-tungsten disulfide heterostructures as an example, evidence of ultrafast and highly efficient interlayer electron transfer and strong interlayer coupling and control. We find that photocarriers injected in tungsten disulfide transfer to graphene in 1 ps and with near-unity efficiency. We also demonstrate that optical properties of tungsten disulfide can be effectively tuned by carriers in graphene. These findings illustrate basic processes required for using van der Waals heterostructures in electronics and photonics.

Third-harmonic generation in ultrathin films of MoS2.

We observe optical third-harmonic generation in atomically thin films of MoS2 and deduce effective third-order nonlinear susceptibilities on the order of 10(-19) m(2)/V(2), which is comparable to that of commonly used semiconductors under resonant conditions. By measuring the susceptibility as a function of light wavelength, we find significant enhancements of the susceptibility by excitonic resonances. The demonstrated third-harmonic generation can be used for nonlinear optical identification of MoS2 atomic layers with high contrast, better distinguishing power of multilayers, and less restrictions to substrate selections. The size of the third-order nonlinear susceptibility suggests feasibility of exploring other types of third-order nonlinear optical effects of MoS2 two-dimensional crystals.

Raman and photocurrent imaging of electrical stress-induced p-n junctions in graphene.

Electrostatically doped graphene p-n junctions can be formed by applying large source-drain and source-gate biases to a graphene field-effect transistor, which results in trapped charges in part of the channel gate oxide. We measure the temperature distribution in situ during the electrical stress and characterize the resulting p-n junctions by Raman spectroscopy and photocurrent microscopy. Doping levels, the size of the doped graphene segments, and the abruptness of the p-n junctions are all extracted. Additional voltage probes can limit the length of the doped segments by acting as heat sinks. The spatial location of the identified potential steps coincides with the position where a photocurrent is generated, confirming the creation of p-n junctions.

Thermal infrared emission from biased graphene.

The high carrier mobility and thermal conductivity of graphene make it a candidate material for future high-speed electronic devices. Although the thermal behaviour of high-speed devices can limit their performance, the thermal properties of graphene devices remain incompletely understood. Here, we show that spatially resolved thermal radiation from biased graphene transistors can be used to extract the temperature distribution, carrier densities and spatial location of the Dirac point in the graphene channel. The graphene exhibits a temperature maximum with a location that can be controlled by the gate voltage. Stationary hot spots are also observed. Infrared emission represents a convenient and non-invasive characterization tool for graphene devices.

Atomic-scale mass sensing using carbon nanotube resonators.

Ultraminiaturized mass spectrometers are highly sought-after tools, with numerous applications in areas such as environmental protection, exploration, and drug development. We realize atomic scale mass sensing using doubly clamped suspended carbon nanotube nanomechanical resonators, in which their single-electron transistor properties allows self-detection of the nanotube vibration. We use the detection of shifts in the resonance frequency of the nanotubes to sense and determine the inertial mass of atoms as well as the mass of the nanotube. This highly sensitive mass detection capability may eventually enable applications such as on-chip detection, analysis, and identification of compounds.