Alex Schenk

A graphene field-effect transistor as a molecule-specific probe of DNA nucleobases

Fast and reliable DNA sequencing is a long-standing target in biomedical research. Recent advances in graphene-based electrical sensors have demonstrated their unprecedented sensitivity to adsorbed molecules, which holds great promise for label-free DNA sequencing technology. To date, the proposed sequencing approaches rely on the ability of graphene electric devices to probe molecular-specific interactions with a graphene surface. Here we experimentally demonstrate the use of graphene field-effect transistors (GFETs) as probes of the presence of a layer of individual DNA nucleobases adsorbed on the graphene surface. We show that GFETs are able to measure distinct coverage-dependent conductance signatures upon adsorption of the four different DNA nucleobases; a result that can be attributed to the formation of an interface dipole field. Comparison between experimental GFET results and synchrotron-based material analysis allowed prediction of the ultimate device sensitivity, and assessment of the feasibility of single nucleobase sensing with graphene.

Surface transfer doping of hydrogen-terminated diamond by C60F48: Energy level scheme and doping efficiency

Surface sensitive C1s core level photoelectron spectroscopy was used to examine the electronic properties of C(60)F(48) molecules on the C(100):H surface. An upward band bending of 0.74 eV in response to surface transfer doping by fluorofullerene molecules is measured. Two distinct molecular charge states of C(60)F(48) are identified and their relative concentration determined as a function of coverage. One corresponds to ionized molecules that participate in surface charge transfer and the other to neutral molecules that do not. The position of the lowest unoccupied molecular orbital of neutral C(60)F(48) which is the relevant acceptor level for transfer doping lies initially 0.6 eV below the valence band maximum and shifts upwards in the course of transfer doping by up to 0.43 eV due to a doping induced surface dipole. This upward shift in conjunction with the band bending determines the occupation of the acceptor level and limits the ultimately achievable hole concentration with C(60)F(48) as a surface acceptor to values close to 10(13) cm(-2) as reported in the literature.

Air-Stable Electron Depletion of Bi2Se3 Using Molybdenum Trioxide into the Topological Regime

We perform high-resolution photoelectron spectroscopy on in situ cleaved topological insulator Bi2Se3 single crystals and in situ transport measurements on Bi2Se3 films grown by molecular beam epitaxy. We demonstrate efficient electron depletion of Bi2Se3 via vacuum deposition of molecular MoO3, lowering the surface Fermi energy to within ∼100 meV of the Dirac point, well into the topological regime. A 100 nm MoO3 film provides an air-stable doping and passivation layer.

Surface transfer doping of diamond with a molecular heterojunction

Surface conductivity and C1s core level measurements were employed to show that surface transfer doping of hydrogen-terminated diamond C(100) can be achieved with a molecular heterojunction formed with C60F48 and an intralayer of zinc-tetraphenylporphyrin. Measurement of the shift in the diamond Fermi energy shows that the zinc-tetraphenylporphyrin (ZnTPP) layer modifies the C60F48–diamond interaction, modulating the extent of charge transfer between the diamond and the fluorofullerene. In contrast to the case of C60F48 acceptors, the presence of a ZnTPP layer prevents the formation of air-induced surface conductivity, showing that the intralayer acts to selectively separate these two doping channels.

Formation of a silicon terminated (100) diamond surface

We report the preparation of an ordered silicon terminated diamond (100) surface with a two domain 3 × 1 reconstruction as determined by low energy electron diffraction. Based on the dimensions of the surface unit cell and on chemical information provided by core level photoemission spectra, a model for the structure is proposed. The termination should provide a homogeneous, nuclear, and electron spin-free surface for the development of future near-surface diamond quantum device architectures.

High resolution core level spectroscopy of hydrogen-terminated (1 0 0) diamond.

Synchrotron-based photoelectron spectroscopy experiments are presented that address a long standing inconsistency in the treatment of the C1s core level of hydrogen terminated (1 0 0) diamond. Through a comparison of surface and bulk sensitive measurements we show that there is a surface related core level component to lower binding energy of the bulk diamond component; this component has a chemical shift of [Formula: see text] eV which has been attributed to carbon atoms which are part of the hydrogen termination. Additionally, our results indicate that the asymmetry of the hydrogen terminated (1 0 0) diamond C1s core level is an intrinsic aspect of the bulk diamond peak which we have attributed to sub-surface carbon layers.

P-type surface transfer doping of oxidised silicon terminated (100) diamond

High-resolution core-level photoemission was used to examine the interaction between the oxidised silicon-terminated diamond (100) surface and the molecular acceptor MoO3. An observed downward shift in the Fermi level position, accompanied by the appearance of two distinct charge states of MoO3, indicates charge transfer from the surface into the MoO3 adlayer in the form of surface transfer doping with a concurrent accumulation of holes in the diamond.

Oxidation of the silicon terminated (1 0 0) diamond surface

The oxidation of the silicon terminated (1 0 0) diamond surface is investigated with a combination of high resolution photoelectron spectroscopy, low energy electron diffraction and near edge x-ray absorption fine structure spectroscopy. The effects of molecular [Formula: see text] and [Formula: see text] dosing under UHV conditions, as well as exposure to ambient conditions, have been explored. Our findings indicate that the choice of oxidant has little influence over the resulting surface chemistry, and we attribute approximately 85% of the surface oxygen to a peroxide-bridging arrangement. Additionally, oxidation does not alter the silicon-carbon bonding at the surface and therefore the [Formula: see text] reconstruction is still present.

Germanium terminated (1 0 0) diamond

An ordered germanium terminated (1 0 0) diamond surface has been formed and characterised using a combination of low energy electron diffraction and synchrotron-based core level photoemission spectroscopy. A number of preparation methods are explored, in each case inducing a two domain [Formula: see text] surface reconstruction. The surface becomes saturated with bonded germanium such that each [Formula: see text] unit cell hosts 1.26 Ge atoms on average, and possesses a negative electron affinity of  -0.71 eV.

Air-stable doping of Bi 2 Se 3 by MoO 3 into the topological regime

We study vacuum deposition of molecular MoO₃ as an acceptor surface transfer dopant for topological insulator Bi₂Se₃. We perform high-resolution photoelectron spectroscopy on in-situ cleaved Bi₂Se3 single crystals and in-situ transport measurements on Bi₂Se₃ films grown by molecular beam epitaxy. MoO₃ is an efficient acceptor, lowering the surface Fermi energy to within -100 meV of the Dirac point, in the topological regime. A 100 nm MoO₃ film provides an air-stable doping and passivation layer.