Electronic structure, chemical bonding features, and electron charge density of the double-cubane single crystal [ Sb 7 S 8 Br 2 ] ( AlCl 4 ) 3 Appl. Phys. Lett. 98, 201903 (2011); 10.1063/1.3583674Structural characterization and electron-energy-loss spectroscopic study of pulsed laser deposited Li Nb O 3 films on a -sapphire Black phosphorus, a layered two-dimensional crystal with tunable electronic properties and high hole mobility, is quickly emerging as a promising candidate for future electronic and photonic devices. Although theoretical studies using ab initio calculations have tried to predict its atomic and electronic structure, uncertainty in its fundamental properties due to a lack of clear experimental evidence continues to stymie our full understanding and application of this novel material. In this work, aberration-corrected scanning transmission electron microscopy and ab initio calculations are used to study the crystal structure of few-layer black phosphorus. Directly interpretable annular dark-field images provide a three-dimensional atomic-resolution view of this layered material in which its stacking order and all three lattice parameters can be unambiguously identified. In addition, electron energy-loss spectroscopy (EELS) is used to measure the conduction band density of states of black phosphorus, which agrees well with the results of density functional theory calculations performed for the experimentally determined crystal. Furthermore, experimental EELS measurements of interband transitions and surface plasmon excitations are also consistent with simulated results. Finally, the effects of oxidation on both the atomic and electronic structure of black phosphorus are analyzed to explain observed device degradation. The transformation of black phosphorus into amorphous PO 3 or H 3 PO 3 during oxidation may ultimately be responsible for the degradation of devices exposed to atmosphere over time.

We report record contact resistance and transconductance in locally back-gated black phosphorus p-MOSFETs with 7-nm-thick HfO2 gate dielectrics. Devices with effective gate lengths, Leff, from 0.55 m to 0.17 m were characterized and shown to have contact resistance values as low as of 1.14 + 0.05 -mm. In addition, devices with Leff = 0.17 m displayed extrinsic transconductance exceeding 250 S/m and on-state current approaching 300 A/m.

The effect of thickness, temperature, and source-drain bias voltage, V(DS), on the subthreshold slope, SS, and off-state properties of black phosphorus (BP) field-effect transistors is reported. Locally back-gated p-MOSFETs with thin HfO2 gate dielectrics were analyzed using exfoliated BP layers ranging in thickness from ∼4 to 14 nm. SS was found to degrade with increasing V(DS) and to a greater extent in thicker flakes. In one of the thinnest devices, SS values as low as 126 mV/decade were achieved at V(DS) = -0.1 V, and the devices displayed record performance at V(DS) = -1.0 V with SS = 161 mV/decade and on-to-off current ratio of 2.84 × 10(3) within a 1 V gate bias window. A one-dimensional transport model has been utilized to extract the band gap, interface state density, and the work function of the metal contacts. The model shows that SS degradation in BP MOSFETs occurs due to the ambipolar turn on of the carriers injected at the drain before the onset of purely thermionic-limited transport at the source. The model is further utilized to provide design guidelines for achieving ideal SS and meet off-state leakage targets, and it is found that band edge work functions and thin flakes are required for ideal operation at high V(DS). This work represents a comprehensive analysis of the fundamental performance limitations of Schottky-contacted BP MOSFETs under realistic operating conditions.

Understanding the interactions of ambient molecules with graphene and adjacent dielectrics is of fundamental importance for a range of graphene-based devices, particularly sensors, where such interactions could influence the operation of the device. It is well-known that water can be trapped underneath graphene and its host substrate, however, the electrical effect of water beneath graphene and the dynamics of how it changes with different ambient conditions has not been quantified. Here, using a metal-oxide-graphene variable-capacitor (varactor) structure, we show that graphene can be used to capacitively sense the intercalation of water between graphene and HfO2 and that this process is reversible on a fast time scale. Atomic force microscopy is used to confirm the intercalation and quantify the displacement of graphene as a function of humidity.Density functional theory simulations are used to quantify the displacement of graphene induced by intercalated water and also explain the observed Dirac point shifts as being due to the combined effect of water and oxygen on the carrier concentration in the graphene. Finally, molecular dynamics simulations indicate that a likely mechanism for the intercalation involves adsorption and lateral diffusion of water molecules beneath the graphene. § Equal contribution Keywords: graphene, sensor, varactor, capacitance, water 2 The successful exfoliation of single-layer graphene and subsequent development of chemical vapor deposition (CVD) for producing large-area graphene sheets has resulted in many interesting device applications. Its use in field-effect transistors, 1 mixers, 2 optical modulators, 3 photodetectors, 4 and a wide variety of chemical sensors is of particular note. [5][6][7][8][9] In nearly all of these device concepts, the intimate interactions between graphene and adjacent dielectrics is critical, yet has not been explored in detail. For instance, it has been shown previously, using density functional theory (DFT) simulations, that the equilibrium distance between graphene and HfO2 is 0.30 nm. 10 However, it has also been shown that this equilibrium distance can change in the presence of defects on graphene or in the adjacent dielectrics, as the defects create bonding sites that result in stronger coupling between graphene and HfO2. 10 The intimate surface interactions can become even more complex with the introduction of small molecules such as H2O, 11 which can often be trapped between the graphene and the adjacent surface. Previously, atomic-force-microscopy (AFM) studies have shown that exfoliated graphene on mica can visualize the trapped water underneath due to the displacement of graphene. 12 However, to date, these trapped molecules have only been probed using physical analysis techniques such as AFM. 12,13 It would be extremely useful if such molecular interactions could be probed using electrical techniques, as such methods could allow a greatly improved understanding of the dynamics of these processes.We have recently proposed a capacitanc...

The operation of an integrated two-dimensional complementary metal-oxide-semiconductor inverter with well-matched input/output voltages is reported. The circuit combines a few-layer MoS2 n-MOSFET and a black phosphorus (BP) p-MOSFET fabricated using a common local backgate electrode with thin (20 nm) HfO2 gate dielectric. The constituent devices have linear threshold voltages of 0.8 V and +0.8 V and produce peak transconductances of 16 S/m and 41 S/m for the MoS2 n-MOSFET and BP p-MOSFET, respectively. The inverter shows a voltage gain of 3.5 at a supply voltage, VDD = 2.5 V, and has peak switching current of 108 A and off-state current of 8.4 A (2.4 A) at VIN = 0 (VIN = 2.5 V). In addition, the inverter has voltage gain greater than unity for VDD > 0.5 V, has open butterfly curves for VDD > 1 V, and achieves static noise margin over 500 mV at VDD = 2.5 V. The voltage gain was found to be insensitive to temperature between 270 K and 340 K, and AC large and small-signal operation was demonstrated at frequencies up to 100 kHz. The demonstration of a complementary 2D inverter which operates in a symmetric voltage window suitable for driving a subsequent logic stage is a significant step forward in developing practical applications for devices based upon 2D materials.