Charge Transport through Carbon Nanomembranes

Abstract

Molecular junctions incorporating pristine and cross-linked aromatic self-assembled monolayers (SAMs) are fabricated and investigated. A two-terminal setup composed of a eutectic Ga–In (EGaIn) top electrode and the gold substrate on which SAMs are prepared as a bottom electrode was used to characterize the charge transport. SAMs of phenylthiol (PT), biphenylthiol (BPT), p-terphenylthiol (TPT), and p-quaterphenylthiol (QPT) are then irradiated with low-energy electrons and converted into carbon nanomembranes (CNMs). A comparison of charge transport through SAMs and CNMs reveals a decrease of conductance of CNM-based junctions by 1 order of magnitude, as well as a conversion of asymmetric junctions with SAMs into symmetric junctions with CNMs, which could be attributed to the decoupling of CNMs from the Au substrate and the partial loss of aromaticity of CNMs after irradiation. Transition voltage spectroscopy (TVS) was also employed to investigate both types of junctions. We observe the length-dependent behavior of transition voltages in both systems and a reduction of transition voltages of CNM-based junctions in comparison to SAM-based junctions.

Directional Excitation of Surface Plasmon Polaritons via Molecular Through-Bond Tunneling across Double-Barrier Tunnel Junctions

“…It has been suggested that molecular junctions can be seen as “series tunneling junctions” where each molecular component forms a distinct part of the tunnel junction. − According to this view of molecular junctions, here, the alkyl chain and BPh units pose two distinct tunneling barriers in series where the tunneling barrier widths are defined by the lengths of the alkyl chain (which depends on n ), d alk , and the BPh unit, d BPh , and tunneling barrier heights are defined by the alkyl chain, φ alk , and BPh unit, φ BPh (Figure c). Thus, in principle, the junctions are double-barrier junctions with the corresponding tunneling decay coefficients along the alkyl chain, β alk , and BPh units, β BPh , given by eq In general, the aromatic part poses a lower φ than the aliphatic molecules (as indicated in Figure c); consequently, aromatic molecules have typical values of β in the range of 0.2–0.4 Å –1 , − whereas aliphatic molecules have β of 0.8 Å –1 . In addition, others have suggested that intermolecular tunneling (where charges tunnel from one molecule to another) may be important for molecules with large tilt angles. − In case the double-barrier picture holds, the tunneling rates are determined by the BPh units for small values of n , and the red arrows in Figure a indicate that the tunneling direction should be determined by the tilt angle of the BPh units, which, in turn, should influence the directional plasmon launching as indicated by the blue arrows.…”

Large-Area All-Carbon Nanocapacitors from Graphene and Carbon Nanomembranes

“…SAMs also cannot grow on top of each other, so their layer thickness is limited to the length of a molecule (typically 1–2 nm). To overcome these limits, we utilize carbon nanomembranes (CNMs) from cross-linked aromatic SAMs via electron irradiation. − The underlying mechanisms of cross-linking have been investigated using different surface-sensitive analytical techniques. − The electronic transport through CNMs was studied by conductive probe atomic force microscope and eutectic Ga–In top contacts, , which demonstrates the potential of CNMs for use as a dielectric material. Recently, CNMs prepared from p -terphenylthiol were found to possess a high permeance and selectivity particularly for water molecules due to the presence of subnanometer channels .…”

“…15−17 The underlying mechanisms of cross-linking have been investigated using different surface-sensitive analytical techniques. 18−21 The electronic transport through CNMs was studied by conductive probe atomic force microscope and eutectic Ga−In top contacts, 22,23 which demonstrates the potential of CNMs for use as a dielectric material. Recently, CNMs prepared from pterphenylthiol were found to possess a high permeance and selectivity particularly for water molecules due to the presence of subnanometer channels.…”

Bottom-Up Synthesis of Graphene Monolayers with Tunable Crystallinity and Porosity

“…In the first growth step, Figure a (i), 4-(4-thiophenyl)­pyridine (TPP) (b), 4-(1 H -pyrrole-1-yl)­thiophenol (PTP) (c), or 4-(2,5-dimethyl-1 H -pyrrole-1-yl)­thiophenol (DPTP) (d) compounds form a SAM on a copper substrate by vapor deposition (VD) under vacuum. In the second step, Figure a (ii), low-energy electron irradiation induced cross-linking converts the SAM into a molecular nanosheet: a carbon nanomembrane (CNM). − CNMs have been fabricated from different molecules, and their structure and mechanical, optical, and electrical behavior can be engineered in this way. − The vacuum pyrolysis results in their conversion into graphene. ,− For the N-containing molecules (TPP, PTP, DPTP, Figure b) investigated in this study, we show that for a low pyrolysis temperature T p 1 nitrogen-doped nanocrystalline graphene is formed, Figure a (iii). By increasing the temperature to T p 2 , the recrystallization of the graphene sheet leads to the formation of nanopores by extrusion of the nitrogen atoms, Figure a (iv).…”