Graphene-based photonic devices, such as ultrafast photodetectors, optical modulators and tunable surface plasmon polariton devices, have experienced rapid development in recent years 1-6 because they benefit greatly from graphene's strong field-controlled optical response 7,8 . Here, we demonstrate a graphene/silicon-heterostructure photodiode formed by integrating graphene onto a silicon optical waveguide on a siliconon-insulator (SOI) with a near to mid-infrared operational range. The waveguide enables absorption of evanescent light that propagates parallel to the graphene sheet, which results in a responsivity as high as 0.13 A W 21 at a 1.5 V bias for 2.75 mm light at room temperature. A photocurrent dependence on bias polarity was observed and attributed to two distinct mechanisms for optical absorption, that is, direct and indirect transitions in graphene at 1.55 mm and 2.75 mm, respectively. Our result demonstrates the use of in-plane absorption in a graphene-monolayer structure and the feasibility of exploiting indirect transitions in graphene/siliconheterostructure waveguides for mid-infrared detection.Graphene, the first truly two-dimensional material, demonstrates unique electronic and optical properties that have stimulated enormous scientific interest 9 . The exceptional optical absorption of graphene, in particular its extremely broad spectrum, stems from its unique band structure 10,11 . This enables the design of a novel photodetector that can cover the mid-infrared range from 2 mm to 8 mm for potential applications in environment monitoring, spectroscopy, kinetic study of drug delivery, and so on 12,13 . State-of-the-art midinfrared detectors employ low-bandgap semiconductors such as HgCdTe alloys 14,15 or quantum-well and quantum-dot structures on III-V materials 16,17 . However, these materials are difficult to grow, and their operation at room temperature is still a challenge 17 . In contrast, graphene photodetectors can be operated at room temperature, which makes them an attractive alternative for midinfrared detection.Photoconductivity in graphene devices has been studied widely across the spectral range from visible light 18,19 to telecommunication bands 1,20 . By optimizing the device geometry, 40 GHz bandwidth 1 and 10 Gbit s 21 data-rate detection 20 have been demonstrated at room temperature, and a graphene-based detector with a possible operation frequency up to 500 GHz is predicted 3 . However, responsivities were limited to several mA W 21 for early demonstrations of graphene-based photodetectors because of the relatively low (2.3%) optical absorption of normal incident light by monolayer graphene 21 . Therefore, recent studies have been aimed at coupling graphene with more-sophisticated optical structures, such as graphene-quantum dot hybrid systems 22 , graphene coupled with optical resonant cavities 23,24 and utilization of in-plane (rather than normal incident light) evanescent field interaction between light in an optical waveguide/plasmonic antenna and graphene 3,25-30 .Suspe...

Two-dimensional atomic crystals are extensively studied in recent years due to their exciting physics and device applications. However, a molecular counterpart, with scalable processability and competitive device performance, is still challenging. Here, we demonstrate that high-quality few-layer dioctylbenzothienobenzothiophene molecular crystals can be grown on graphene or boron nitride substrate via van der Waals epitaxy, with precisely controlled thickness down to monolayer, large-area single crystal, low process temperature and patterning capability. The crystalline layers are atomically smooth and effectively decoupled from the substrate due to weak van der Waals interactions, affording a pristine interface for high-performance organic transistors. As a result, monolayer dioctylbenzothienobenzothiophene molecular crystal field-effect transistors on boron nitride show record-high carrier mobility up to 10 cm 2 V À 1 s À 1 and aggressively scaled saturation voltage B1 V. Our work unveils an exciting new class of two-dimensional molecular materials for electronic and optoelectronic applications.

One of the basic assumptions in organic field-effect transistors, the most fundamental device unit in organic electronics, is that charge transport occurs two-dimensionally in the first few molecular layers near the dielectric interface.Although the mobility of bulk organic semiconductors has increased dramatically, direct probing of intrinsic charge transport in the two-dimensional limit has not been possible due to excessive disorders and traps in ultrathin organic thin films. Here, highly ordered mono-to tetra-layer pentacene crystals are realized by van der Waals (vdW) epitaxy on hexagonal BN. We find that the charge transport is dominated by hopping in the first conductive layer, but transforms to band-like in subsequent layers.Such abrupt phase transition is attributed to strong modulation of the molecular packing by interfacial vdW interactions, as corroborated by quantitative structural characterization and density functional theory calculations. The structural modulation becomes negligible beyond the second conductive layer, leading to a mobility saturation thickness of only ~3nm. Highly ordered organic ultrathin films provide a platform for new physics and device structures (such as heterostructures and quantum wells) that are not possible in conventional bulk crystals. 3Organic field-effect transistors (OFETs) offer unique advantages of low cost, lightweight and flexibility and are widely used in electronics and display industry.While the mobility of bulk organic semiconductors has increased dramatically [1][2][3], an outstanding issue is to directly examine the structure-property relationship at the semiconductor-dielectric interface [4], where charge transport actually occurs [5][6][7].Ultrathin organic semiconductors down to few-nanometre thickness are often dominated by traps and disorders and far away from intrinsic transport regime [8][9][10].Another challenge in organic electronics is the development of layer-by-layer epitaxy with the precision similar to molecular beam epitaxy in their inorganic counterparts [11]. These challenges may be alleviated if molecular crystals are processed into large-area, highly crystalline monolayers. Such 2D form factor will also bring about new applications such as nanoporous membranes and insulating dielectrics [12,13].Several recent breakthroughs in various types of 2D organic materials such as polymers [14,15], oligomers [16] and covalent organic frameworks [17] have already shown great promises along this direction. However, one of the most fundamental questions regarding the nature of charge transport at the 2D limit has not been addressed. In this work, we study the benchmark molecule pentacene epitaxially crystallized on BN substrate because of its high mobility and simple structure to model. The highly clean system allows us to provide the first definitive scenario of how molecular packing and charge transport are modulated near the interface, without being dominated by extrinsic factors. Our results suggest the possibility of band-like transport...

The exfoliation and identification of the two-dimensional (2D) single atomic layer of carbon have opened the opportunity to explore graphene and related 2D materials due to their unique properties. 2D materials are regarded as one of the most exciting solutions for next generation electronics and optoelectronics in the technological evolution of semiconductor technology. In this review, we focus on the core concept of "structure-property relationships" to explain the state-of-the-art of 2D materials and summarize the unique electrical and light-matter interaction properties in 2D materials. Based on this, we discuss and analyze the structural properties of 2D materials, such as defects and dopants, the number of layers, composition, phase, strain, and other structural characteristics, which could significantly alter the properties of 2D materials and hence affect the performance of semiconductor devices. In particular, the building blocks principles and potential electronic and optoelectronic applications based on 2D materials are explained and illustrated. Indeed, 2D materials and related heterostructures offer the promise for challenging the existing technologies and providing the chance to have social impact. More efforts are expected to propel this exciting field forward.

The piezoelectric effect is widely applied in pressure sensors for the detection of dynamic signals. However, these piezoelectric-induced pressure sensors have challenges in measuring static signals that are based on the transient flow of electrons in an external load as driven by the piezopotential arisen from dynamic stress. Here, we present a pressure sensor with nanowires/graphene heterostructures for static measurements based on the synergistic mechanisms between strain-induced polarization charges in piezoelectric nanowires and the caused change of carrier scattering in graphene. Compared to the conventional piezoelectric nanowire or graphene pressure sensors, this sensor is capable of measuring static pressures with a sensitivity of up to 9.4 × 10 kPa and a fast response time down to 5-7 ms. This demonstration of pressure sensors shows great potential in the applications of electronic skin and wearable devices.

We present a self-powered, high-performance graphene-enhanced ultraviolet silicon Schottky photodetector. Different from traditional transparent electrodes, such as indium tin oxides or ultra-thin metals, the unique ultraviolet absorption property of graphene leads to long carrier life time of hot electrons that can contribute to the photocurrent or potential carrier-multiplication. Our proposed structure boosts the internal quantum efficiency over 100%, approaching the upper-limit of silicon-based ultraviolet photodetector. In the near-ultraviolet and mid-ultraviolet spectral region, the proposed ultraviolet photodetector exhibits high performance at zero-biasing (self-powered) mode, including high photo-responsivity (0.2 A W −1 ), fast time response (5 ns), high specific detectivity (1.6 × 10 13 Jones), and internal quantum efficiency greater than 100%. Further, the photo-responsivity is larger than 0.14 A W −1 in wavelength range from 200 to 400 nm, comparable to that of state-of-the-art Si, GaN, SiC Schottky photodetectors. The photodetectors exhibit stable operations in the ambient condition even 2 years after fabrication, showing great potential in practical applications, such as wearable devices, communication, and "dissipation-less" remote sensor networks.

2D van der Waals (vdW) layered polar crystals sustaining phonon polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications in the mid-infrared to terahertz ranges. To date, 2D vdW crystals with PhPs are only experimentally demonstrated in hexagonal boron nitride (hBN) slabs. For optoelectronic and active photonic applications, semiconductors with tunable charges, finite conductivity, and moderate bandgaps are preferred. Here, PhPs are demonstrated with low loss and ultrahigh electromagnetic field confinements in semiconducting vdW α-MoO . The α-MoO supports strong hyperbolic PhPs in the mid-infrared range, with a damping rate as low as 0.08. The electromagnetic confinements can reach ≈λ /120, which can be tailored by altering the thicknesses of the α-MoO 2D flakes. Furthermore, spatial control over the PhPs is achieved with a metal-ion-intercalation strategy. The results demonstrate α-MoO as a new platform for studying hyperbolic PhPs with tunability, which enable switchable mid-infrared nanophotonic devices.

Solution-processed n-channel organic thin-film transistors (OTFTs) that exhibit a field-effect mobility as high as 11 cm(2) V(-1) s(-1) at room temperature and a band-like temperature dependence of electron mobility are reported. By comparison of solution-processed OTFTs with vacuum-deposited OTFTs of the same organic semiconductor, it is found that grain boundaries are a key factor inhibiting band-like charge transport.