Hunter J. Karmel

Chemically homogeneous and thermally reversible oxidation of epitaxial graphene

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-Processable High Capacitance Hafnia-Organic Self- Assembled Nanodielectrics

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.

Quantitatively enhanced reliability and uniformity of high-κ dielectrics on graphene enabled by self-assembled seeding layers

The full potential of graphene in integrated circuits can only be realized with a reliable ultrathin high-κ top-gate dielectric. Here, we report the first statistical analysis of the breakdown characteristics of dielectrics on graphene, which allows the simultaneous optimization of gate capacitance and the key parameters that describe large-area uniformity and dielectric strength. In particular, vertically heterogeneous and laterally homogeneous Al2O3 and HfO2 stacks grown via atomic-layer deposition and seeded by a molecularly thin perylene-3,4,9,10-tetracarboxylic dianhydride organic monolayer exhibit high uniformities (Weibull shape parameter β > 25) and large breakdown strengths (Weibull scale parameter, E(BD) > 7 MV/cm) that are comparable to control dielectrics grown on Si substrates.

Self-Assembled Two-Dimensional Heteromolecular Nanoporous Molecular Arrays on Epitaxial Graphene

The development of graphene functionalization strategies that simultaneously achieve two-dimensional (2D) spatial periodicity and substrate registry is of critical importance for graphene-based nanoelectronics and related technologies. Here, we demonstrate the generation of a hydrogen-bonded molecularly thin organic heteromolecular nanoporous network on epitaxial graphene on SiC(0001) using room-temperature ultrahigh vacuum scanning tunneling microscopy. In particular, perylenetetracarboxylic diimide (PTCDI) and melamine are intermixed to form a spatially periodic 2D nanoporous network architecture with hexagonal symmetry and a lattice parameter of 3.45 ± 0.10 nm. The resulting adlayer is in registry with the underlying graphene substrate and possesses a characteristic domain size of 40-50 nm. This molecularly defined nanoporous network holds promise as a template for 2D ordered chemical modification of graphene at lengths scales relevant for graphene band structure engineering.

Self-assembled organic monolayers on epitaxial graphene with enhanced structural and thermal stability

Scanning tunnelling microscopy and X-ray reflectivity are used to characterize adlayers of perylenetetracarboxylic diimide (PTCDI) deposited on epitaxial graphene (EG) on SiC(0001). PTCDI adopts a herringbone structural phase on EG/SiC that can accommodate sub-5 nm voids with molecularly defined boundaries and isolated molecular vacancies at room temperature. The PTCDI monolayer remains intact up to substrate temperatures of ~260 °C, thus demonstrating enhanced thermal stability compared to previously studied perylene derivatives on EG/SiC.

Room temperature molecular resolution nanopatterning of cyclopentene monolayers on Si(100) via feedback controlled lithography

Molecularly precise nanopatterning is demonstrated for a saturated organic monolayer on the Si(100) surface using room temperature ultra-high vacuum scanning tunneling microscopy. In particular, feedback controlled lithography enables the clean desorption of individual molecules from a highly-ordered cyclopentene monolayer at moderate negative sample bias, resulting in the exposure of isolated silicon dimers on an otherwise organically passivated surface. The quality and uniformity of the cyclopentene passivation layer is also quantified with X-ray photoelectron spectroscopy following exposure to ambient conditions, revealing that complete formation of the native oxide on silicon is suppressed for time scales exceeding 100 days.