Md. Zakir Hossain

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.

Scanning tunneling microscopy, spectroscopy, and nanolithography of epitaxial graphene chemically modified with aryl moieties

The reduction of diazonium salts has recently been proposed as a robust covalent modification scheme for graphene surfaces. While preliminary studies have provided indirect evidence that this strategy decorates graphene with aryl moieties, the molecular ordering and conformation of the resulting adlayer have not been directly measured. In this Article, we report molecular-resolution characterization of the adlayer formed via the spontaneous reduction of 4-nitrophenyl diazonium (4-NPD) tetrafluoroborate on epitaxial graphene on SiC(0001) using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS). An atomically flat inhomogeneous layer of covalently bonded organic molecules is observed after annealing the chemically treated surface at ∼500 °C in UHV. STM and STS results indicate that the adlayer consists predominantly of aryl oligomers that sterically prevent uniform and complete covalent modification of the graphene surface. The adsorbed species can be selectively desorbed by the STM tip above a threshold sample bias of -5 V and tunneling current of 1 nA, thus enabling the fabrication of a diverse range of graphene nanopatterns at the sub-5 nm length scale.

Self-Directed Chain Reaction by Small Ketones with the Dangling Bond Site on the Si(100)-(2 × 1)-H Surface: Acetophenone, A Unique Example

Using scanning tunneling microscope (STM) at 300 K, we studied the growth of one-dimensional molecular assemblies (molecular lines) on the Si(100)-(2 x 1)-H surface through the chain reaction of small ketone (CH 3COCH 3, PhCOPh, and PhCOCH 3) molecules with dangling bond (DB) sites of the substrate. Acetone and benzophenone show the growth of molecular lines exclusively parallel to the dimer row direction. In contrast, acetophenone molecules show some molecular lines perpendicular, in addition to parallel, to the dimer row direction. Most of the molecular lines perpendicular to the dimer row direction were grown by self-turning the propagation direction of a chain reaction from parallel to perpendicular directions relative to the dimer row. A chiral center created upon adsorption of an acetophenone molecule allows the adsorbed molecules to align with identical as well as alternate enantiomeric forms along the dimer row direction, whereas such variations in molecular arrangement are not observed in the case of acetone and benzophenone molecules. The observed molecular lines growth both parallel and perpendicular to dimer row directions appears to be unique to acetophenone among all the molecules studied to date. Hence, the present study opens new possibility for fabricating one-dimensional molecular assemblies of various compositions in both high-symmetry directions on the Si(100)-(2 x 1)-H surface.

Diels-Alder Reaction on the Clean Diamond (100) 2×1 Surface

The interaction of 1,3-butadiene with the C(100)2?1 surface at 300 K has been studied by electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). EELS studies show that 1,3-butadiene readily chemisorbs on the C(100)2?1 surface by the Diels-Alder reaction ([4+2] cycloaddition) where the surface dimer act as a dienophile. The surface product is found to be stable up to ?1000 K. Orbital symmetry of the reacting species is an important factor in the reaction of unsaturated hydrocarbons with the C(100)2?1 surface.

Surface Phonons, Electronic Structure and Chemical Reactivity of Diamond (100)(2 ×1) Surface

Surface phonons, electronic structure and chemical reactivity of the diamond (100)(2 ×1) surface have been studied using electron energy loss spectroscopy (EELS), thermal desorption spectroscopy (TDS) and low-energy electron diffraction (LEED). Vibrational losses are observed at ~80, 92, 123, 135, 147 and 165 meV for a clean C(100)(2 ×1) surface. The 92 meV loss is assigned to the in-phase bouncing mode of the surface dimers. The origins of the other losses are discussed. Electronic transition is observed at 3.5 eV which is associated with the interband transition between the π and π* surface states. The chemical reactivity of the C(100)(2 ×1) surface towards several gases, H, H2, O, O2, CO, N2O and C2H2, has been investigated at 90 and 300 K. The chemical reactivity of the C(100)(2 ×1) surface towards these gases is compared with that of the Si(100)(2 ×1) surface, and the origin of the difference in the reactivity is discussed.

Adsorbed states of K on the diamond (100) (2 × 1) surface

Abstract The adsorbed states of K on the C(100)(2×1) surface have been studied by electron energy loss spectroscopy (EELS), work function change (Δφ) measurement and thermal desorption spectroscopy (TDS). In the region where the K coverage is less than one-half of a monolayer (θ K ≤0.5), a loss is observed from ∼1.2 eV (θ K =0.2) to 1.0 eV (θ K =0.5); the work function decreases upon K adsorption until reaching a shallow minimum of Δφ=−3.35 eV at θ K =∼0.5; and a desorption peak (β) is observed from ∼825 K (θ K =0.05) to 525 K (θ K =0.5). These results indicate that the K–substrate bond is highly polarized; the 1.2 eV loss is attributed to the electronic transition from the bonding to antibonding states formed at the K–substrate interface. In the region between θ K =0.5 and 1, two losses are observed at 0.7 and 1.4 eV (θ K =0.6); there is only a small increase of the work function; and a desorption peak (α) is observed in addition to the β peak. These results indicate that the K regains its electron and becomes, essentially, neutral. The 1.4-eV loss is ascribed to the transition from the 4s to 4p states of K. The origin of the 0.7-eV loss is discussed. The Δφ and TDS results are analyzed by the depolarization model.

Chemisorption of O2 and CO on the K-modified diamond (100)2 × 1 surface

Abstract The adsorption of O2 and CO molecules on the K-modified C(100) surface has been studied mainly by electron energy loss spectroscopy (EELS) and additionally by thermal desorption spectroscopy (TDS) and low-energy electron diffraction (LEED) at 300 K. Although O2 does not react with the clean C(100) surface, it readily reacts with the K-modified surface. The adsorbed species are characterized by the two loss peaks at 150 and 214 meV. The 150 and 214-meV losses are ascribed to the CO stretch of the COC (ether) and >CO (carbonyl) species which are formed by breaking both σ and π bonds of a surface dimer, respectively. In contrast to Si(100), substrate oxidation mainly occurs at the top layer of C(100). The CO molecule also reacts with the K-modified surface, while it does not react with the clean C(100) surface. The adsorbed species are characterized by the loss peaks at 154 meV with a shoulder at 192 meV. The 154-meV loss is tentatively assigned to the CO stretch of the (C2O2)2−2K+ complex formed on the K-modified C(100) surface. The shoulder at 192 meV is ascribed to the CO stretch of either (C4O4)2−2K+ or >CO, in which the π bond is largely perturbed by the K adatoms.

Dispersive Electronic States of the π-Orbitals Stacking in Single Molecular Lines on the Si(001)-(2×1)-H Surface.

One-dimensional (1D) molecular assemblies have been considered as one of the potential candidates for miniaturized electronic circuits in organic electronics. Here, we present the quantitative experimental measurements of the dispersive electronic feature of 1D benzophenone molecular assemblies on the Si(001)-(2×1)-H. The well-aligned molecular lines and their certain electronic state dispersion were observed by scanning tunneling microscopy (STM) and angle-resolved ultraviolet photoemission spectroscopy (ARUPS), respectively. Density functional theory (DFT) calculations reproduced not only the experimental STM image but also the dispersive features that originated from the stacking phenyl π-orbitals in the molecular assembly. We obtained the effective mass of 2.0me for the hole carrier along the dispersive electronic state, which was comparable to those of the single-crystal molecules widely used in organic electronic applications. These results ensure the one-dimensionally delocalized electronic states in the molecular lines, which is requisitely demanded for a charge-transport wire.