Scanning Tunneling Microscopy, Spectroscopy, and Nanolithography of Epitaxial Graphene Chemically Modified with Aryl Moieties

Abstract

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.

Organometallic chemistry of extended periodic π-electron systems: hexahapto-chromium complexes of graphene and single-walled carbon nanotubes

“…43 Scanning tunneling microscopy provides more direct evidence of chemical functionalization, if well ordered samples from optimized reactions are available. [52][53][54] We have made use of FT-IR spectroscopy in characterizing the products of the reaction of SWNTs, HOPG, and graphene with organometallic precursors, together with Raman microscopy (excitation at  = 532 nm) and absorbance spectroscopy which are well known in carbon materials science. In SWNTs, the inter-band transitions in metallic and semiconducting structures give rise to distinct peaks in the UV-vis-NIR absorbance spectrum and modification of the -conjugation affects these band transitions.…”

Understanding and controlling the substrate effect on graphene electron-transfer chemistry via reactivity imprint lithography

“…The p-doping after reaction has contributions from the covalent bond formation itself and from the non-covalent adsorption of the diazonium cation and oligomers. 12,13,18 The FWHM of the 2D peak (Γ 2D ) is plotted against its position (ω 2D ) in Figure 2d. Since the 2D peak position shifts in opposite directions for electron and hole doping (Figure 2c), the presence of electron-hole puddles significantly smaller than the Raman laser spot size would result in a broadened 2D…”

“…3 The chemical functionalization of graphene is important for enabling these applications, and has been explored via covalent 4,5 and noncovalent [6][7][8] schemes. The functionalization of graphene with aryl diazonium salts 4,[9][10][11][12][13][14][15][16] results in the opening of a band gap 10,13,[17][18][19] and shifting of the Fermi level, 10 which are both desirable for the fabrication of electronic devices. In addition, the functional groups on the diazonium moiety can be tailored by organic chemistry so that various chemical characteristics to be coupled to graphene.…”

Organic semiconductors for device applications: current trends and future prospects

“…Similarly, the covalent attachment of nitrophenyls to graphene sheets [216] introduces a band gap, which can be controlled, making the functionalized graphenes a semiconducting nanomaterial. The reaction with diazonium salts is used for functionalizing different types of graphenes including chemically/thermally converted graphenes, single sheets from cleavage of bulk graphite, as well as epitaxial graphenes [218][219][220][221][222][223]. Another alternative method involving the reaction of benzoyl peroxide with graphene sheets [224], where Ar-ion laser irradiation initiated reaction on graphene deposited on a Si substrate put in a benzoyl peroxide/toluene solution, is used to modify a graphene sheet placed on an OFET device, where, apart from significant decrease in conductivity due to the increase of sp 3 carbon atoms after the covalent addition of phenyl groups, an increase in the level of hole doping is also observed.…”