Van der Waals coupling is emerging as a powerful method to engineer physical properties of atomically thin two-dimensional materials. In coupled graphene-graphene and graphene-boron nitride layers, interesting physical phenomena ranging from Fermi velocity renormalization to Hofstadter's butterfly pattern have been demonstrated. Atomically thin transition metal dichalcogenides, another family of two-dimensional-layered semiconductors, can show distinct coupling phenomena. Here we demonstrate the evolution of interlayer coupling with twist angles in as-grown molybdenum disulfide bilayers. We find that the indirect bandgap size varies appreciably with the stacking configuration: it shows the largest redshift for AA-and AB-stacked bilayers, and a significantly smaller but constant redshift for all other twist angles. Our observations, together with ab initio calculations, reveal that this evolution of interlayer coupling originates from the repulsive steric effects that leads to different interlayer separations between the two molybdenum disulfide layers in different stacking configurations.

No abstract

We investigate the ultrafast terahertz response of electrostatically gated graphene upon optical excitation. We observe that the photoinduced terahertz absorption increases in charge neutral graphene but decreases in highly doped graphene. We show that this transition from semiconductor-like to metal-like response is unique for zero bandgap materials such as graphene. In charge neutral graphene photoexcited hot carriers effectively increase electron and hole densities and increase the conductivity. In highly doped graphene, however, photoexcitation does not change net conducting carrier concentration. Instead, it mainly increases electron scattering rate and reduce the conductivity.

* These two authors contribute equally to this work 2 Two paramount challenges in carbon nanotube research are achieving chiralitycontrolled synthesis and understanding chirality-dependent device physics 1-7 . Highthroughput and in-situ chirality and electronic structural characterization of individual carbon nanotubes is crucial for addressing these challenges. Optical imaging and spectroscopy has unparalleled throughput and specificity 8-14 , but its realization for single nanotubes on substrates or in devices has long been an outstanding challenge.Here we demonstrate video-rate imaging and in-situ spectroscopy of individual carbon nanotubes on various substrates and in functional devices using a novel high-contrast polarization-based optical microscopy. Our technique enables the complete chirality profiling of hundreds of as-grown carbon nanotubes. In addition, we in-situ monitor nanotube electronic structure in active field-effect devices, and observe that high-order nanotube optical resonances are dramatically broadened by electrostatic doping. This unexpected behaviour points to strong interband electron-electron scattering processes that can dominate ultrafast dynamics of excited states in carbon nanotubes.3 Single-walled carbon nanotubes (SWNTs) comprise a large family of tubular carbon structures characterized by different chiral indices (n, m), each having distinct electronic structure and physical properties 1 . They are promising materials for next generation nanoelectronic and nano-photonic devices, including field-effect transistors, light emitters and photocurrent/photovoltaics device [1][2][3][4][5][6][7] . Currently nanotube research faces two outstanding challenges: (1) achieving chirality-controlled nanotube growth and (2) understanding chirality-dependent nanotube device physics. Addressing these challenges requires, respectively, high-throughput determination of nanotube chirality distribution on growth substrates and in-situ characterization of nanotube electronic structure in operating devices.Direct optical imaging and spectroscopy is well suited for these goals [8][9][10][11][12][13][14] , but its realization for single nanotubes on substrates or in devices has been an outstanding challenge due to small nanotube signal and unavoidable environment background. Here we demonstrate for the first time high-throughput real-time optical imaging and broadband spectroscopy of individual nanotubes in devices using a polarization-based microscopy combined with supercontinuum laser illumination. Our technique is generally applicable to semiconducting and metallic nanotubes in various configurations, such as on (transparent or opaque) substrates, between contact electrodes, and under top gates. This is in contrast to strong constraints limiting other prevailing single-tube spectroscopy techniques: single-tube fluorescence spectroscopy only works for isolated semiconducting nanotubes 8 ; Rayleigh scattering requires nanotubes suspended or oil-immersed on transparent substrate 9-12 ; and resonant Ra...

The unusual electronic and optical properties of many electroluminescent and conducting polymers arise from extended conjugation along the polymer backbone, which can also lead to insolubility, aggregation, and gelation. Synthetic efforts to produce an optimal structure require a balance between the persistence length and the effective conjugation length for the successful implementation of these materials in photonic and electronic devices. In this study, we have investigated the solution properties of a group of poly(phenyleneethynylenes) using a variety of light scattering techniques, including polarized and depolarized intensity measurements, dynamic light scattering, and size exclusion chromatography with a multiangle light scattering detector (SEC/LS). Interpretation of light scattering in the presence of absorption, fluorescence, and optical anisotropy is discussed. The molecular weights determined by light scattering encompassed the range from 10 × 105 to 5 × 106, with the root-mean-square radius of gyration as high as 250 nm. The results may be interpreted with a wormlike chain model to yield a persistence length of about 15 nm, so that these high-M polymers are coil-like in solution, rather than “rigid rods”. This persistence length is still expected to be several times larger than the effective conjugation length.

No abstract

Graphene has extremely low mass density and high mechanical strength, key qualities for efficient wide-frequency-response electrostatic audio speaker design. Low mass ensures good high frequency response, while high strength allows for relatively large free-standing diaphragms necessary for effective low frequency response. Here we report on construction and testing of a miniaturized graphene-based electrostatic audio transducer. The speaker/earphone is straightforward in design and operation and has excellent frequency response across the entire audio frequency range (20HZ -20kHz), with performance matching or surpassing commercially available audio earphones.

We present a graphene-based wideband microphone and a related ultrasonic radio that can be used for wireless communication. It is shown that graphene-based acoustic transmitters and receivers have a wide bandwidth, from the audible region (20∼20 kHz) to the ultrasonic region (20 kHz to at least 0.5 MHz). Using the graphene-based components, we demonstrate efficient high-fidelity information transmission using an ultrasonic band centered at 0.3 MHz. The graphene-based microphone is also shown to be capable of directly receiving ultrasound signals generated by bats in the field, and the ultrasonic radio, coupled to electromagnetic (EM) radio, is shown to function as a high-accuracy rangefinder. The ultrasonic radio could serve as a useful addition to wireless communication technology where the propagation of EM waves is difficult.odern wireless communication is based on generating and receiving electromagnetic (EM) waves that span a wide frequency range, from hertz to terahertz, providing abundant band resources and high data transfer rates. There are drawbacks to EM communication, though, including high extinction coefficient for electrically conductive materials and antenna size. However, animals have effectively used acoustic waves for shortrange communication for millions of years. Acoustic wave-based communication, while embodying reduced band resources, can overcome some of the EM difficulties and complement existing wireless technologies. For example, acoustic waves propagate well in conductive materials, and have thus been explored for underwater communication by submarines (1, 2). Marine mammals such as whales and dolphins are known to communicate effectively via acoustic waves. In land-based acoustic wave communication, the audible band is often occupied by human conversations, whereas the subsonic band can be disturbed by moving vehicles and building construction. The ultrasonic band, though having a wide frequency span and often free of disturbance, is rarely exploited for high data rate communication purposes; one possible reason for this is the lack of wide bandwidth ultrasonic generators and receivers. Conventional piezoelectric-based transducers only operate near their resonance frequencies (3, 4), preventing use in communications where wider bandwidth is essential for embedding information streams.In a conventional acoustic transducer such as a microphone, air pressure variations from a sound wave induce motion of a suspended diaphragm; this motion is in turn converted to an electrical signal via Faraday induction (using a magnet and coil) or capacitively. The areal mass density of the diaphragm sets an upper limit on the frequency response (FR) of the microphone. In the human auditory system, the diaphragm (eardrum) is relatively thick (∼100 μm), limiting flat FR to ∼2 kHz and ultimate detection to ∼20 kHz (5, 6). In bats the eardrums are thinner, allowing them to hear reflected echolocation calls up to ∼200 kHz (7-9). Diaphragms in high-end commercial microphones can be engineered to provide fla...