With its exceptional charge mobility, graphene holds great promise for applications in next-generation electronics. In an effort to tailor its properties and interfacial characteristics, the chemical functionalization of graphene is being actively pursued. The oxidation of graphene via the Hummers method is most widely used in current studies, although the chemical inhomogeneity and irreversibility of the resulting graphene oxide compromises its use in high-performance devices. Here, we present an alternative approach for oxidizing epitaxial graphene using atomic oxygen in ultrahigh vacuum. Atomic-resolution characterization with scanning tunnelling microscopy is quantitatively compared to density functional theory, showing that ultrahigh-vacuum oxidization results in uniform epoxy functionalization. Furthermore, this oxidation is shown to be fully reversible at temperatures as low as 260 °C using scanning tunnelling microscopy and spectroscopic techniques. In this manner, ultrahigh-vacuum oxidation overcomes the limitations of Hummers-method graphene oxide, thus creating new opportunities for the study and application of chemically functionalized graphene.

Ambient and solution-processable, low-leakage, high capacitance gate dielectrics are of great interest for advances in low-cost, flexible, thin-film transistor circuitry. Here we report a new hafnium oxide-organic self-assembled nanodielectric (Hf-SAND) material consisting of regular, alternating π-electron layers of 4-[[4-[bis(2-hydroxyethyl)amino]phenyl]diazenyl]-1-[4-(diethoxyphosphoryl) benzyl]pyridinium bromide) (PAE) and HfO2 nanolayers. These Hf-SAND multilayers are grown from solution in ambient with processing temperatures ≤150 °C and are characterized by AFM, XPS, X-ray reflectivity (2.3 nm repeat spacing), X-ray fluorescence, cross-sectional TEM, and capacitance measurements. The latter yield the largest capacitance to date (1.1 μF/cm(2)) for a solid-state solution-processed hybrid inorganic-organic gate dielectric, with effective oxide thickness values as low as 3.1 nm and have gate leakage <10(-7) A/cm(2) at ±2 MV/cm using photolithographically patterned contacts (0.04 mm(2)). The sizable Hf-SAND capacitances are attributed to relatively large PAE coverages on the HfO2 layers, confirmed by X-ray reflectivity and X-ray fluorescence. Random network semiconductor-enriched single-walled carbon nanotube transistors were used to test Hf-SAND utility in electronics and afforded record on-state transconductances (5.5 mS) at large on:off current ratios (I(ON):I(OFF)) of ~10(5) with steep 150 mV/dec subthreshold swings and intrinsic field-effect mobilities up to 137 cm(2)/(V s). Large-area devices (>0.2 mm(2)) on Hf-SAND (6.5 nm thick) achieve mA on currents at ultralow gate voltages (<1 V) with low gate leakage (<2 nA), highlighting the defect-free and conformal nature of this nanodielectric. High-temperature annealing in ambient (400 °C) has limited impact on Hf-SAND leakage densities (<10(-6) A/cm(2) at ±2 V) and enhances Hf-SAND multilayer capacitance densities to nearly 1 μF/cm(2), demonstrating excellent compatibility with device postprocessing methodologies. These results represent a significant advance in hybrid organic-inorganic dielectric materials and suggest synthetic routes to even higher capacitance materials useful for unconventional electronics.

The solid electrolyte interphase (SEI) plays a critical role in the performance and safety of Li-ion batteries, but the crystal structure of the materials formed have not been previously studied. We employ the model system of epitaxial graphene on SiC to provide a well-defined graphitic surface to study the crystallinity and texture formation in the SEI. We observe, via in situ synchrotron X-ray scattering, the formation and growth of LiF crystallites at the graphene surface, which increase in size with lithiation dose and are textured such that the LiF (002) planes are approximately parallel to the graphene sheets. Furthermore, X-ray photoelectron spectroscopy (XPS) reveals the composition of the SEI formed in this system to consist of LiF and organic compounds similar to those found previously on graphite. SEI components, other than LiF, do not produce X-ray diffraction peaks and are categorized as amorphous. From high-resolution transmission electron microscopy, the LiF crystallites are seen in near proximity to the graphene surface along with additional apparently amorphous material, which is likely to be other SEI components detected by XPS and/or misoriented LiF. This new understanding that LiF crystallites grow on the graphene surface with strong texturing will assist future efforts to model and engineer the SEI formed on graphitic materials.

Due to the high cost, brittle nature, and suboptimal electronic and chemical properties of indium tin oxide (ITO), [ 1 − 4 ] alternative transparent conducting anode materials have played an increasingly important role in organic photovoltaic (OPV) device research. For example, thin fi lms of single-walled carbon nanotubes (SWNTs) have been identifi ed as a promising option due to their excellent electronic properties, solution processability, elemental abundance, environmental stability, and robust mechanical fl exibility. [ 5 − 7 ] Recent reports have demonstrated OPVs incorporating SWNT fi lms as the transparent anode, with the primary barrier to greater effi ciencies being the relatively high sheet resistance of the SWNT fi lm. [ 8 − 10 ] One of the common methods employed to overcome this obstacle has been chemical doping of the fi lms prior to device fabrication, either intentionally or as a byproduct of roughnessreducing acid treatments. In particular, the adsorption of electron-withdrawing species both lowers the SWNT fi lm sheet resistance and bleaches the primary peaks in the optical absorption spectrum, thereby increasing the fi lm transparency. [ 11 − 14 ] However, this chemical doping strategy introduces limited environmental stability, which compromises performance in many device applications. [ 5 , 15 ] Additionally, because previous studies of SWNT-based OPV anodes have employed thin fi lms formulated from electronically polydisperse SWNT mixtures, the role and relative importance of metallic versus semiconducting SWNTs has not been clarifi ed.Herein, we present the use of electronically monodisperse arc discharge SWNTs, sorted via density gradient ultracentrifugation (DGU), [ 7 , 16 ] as the transparent anode material in OPVs. By varying the ratio of semiconducting and metallic species in the SWNT thin fi lms, we fi nd that a composition of 70% or greater metallic SWNTs affords 50× higher OPV power conversion effi ciency (PCE) than monodisperse semiconducting SWNT thin fi lms. Analysis of the stability after chemical doping with nitric acid, which is used to lower the fi lm roughness, indicates that the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electron-blocking layer reverses the effects of the doping process and reduces the SWNT electrical conductivity. X-ray photoelectron spectroscopy (XPS) further reveals that nitric oxide (NO) is the primary adsorbed dopant species that is removed upon PEDOT:PSS deposition. Since the electronic and optical properties of metallic SWNTs are less affected by chemical doping, they remain effective transparent conductors following OPV fabrication, thus explaining the 50× difference in device PCE for metallic versus semiconducting SWNT-enriched anodes. Overall, this study establishes that SWNT chemical doping is incompatible with PEDOT:PSS, thus demonstrating the importance of metallic SWNT-enriched materials in OPV anodes.Previous studies recognized the importance of minimizing the roughness of SWNT thin fi lms in organic electron...

Atomic-layer 2D crystals have unique properties that can be significantly modified through interaction with an underlying support. For epitaxial graphene on SiC(0001), the interface strongly influences the electronic properties of the overlaying graphene. We demonstrate a novel combination of x-ray scattering and spectroscopy for studying the complexities of such a buried interface structure. This approach employs x-ray standing wave-excited photoelectron spectroscopy in conjunction with x-ray reflectivity to produce a highly resolved chemically sensitive atomic profile for the terminal substrate bilayers, interface, and graphene layers along the SiC[0001] direction.