Jan A.
Mol‡
*a,
Chit Siong
Lau‡
a,
Wilfred J. M.
Lewis
b,
Hatef
Sadeghi
c,
Cecile
Roche
b,
Arjen
Cnossen
b,
Jamie H.
Warner
a,
Colin J.
Lambert
c,
Harry L.
Anderson
b and
G. Andrew D.
Briggs
a
aDepartment of Materials, University of Oxford, 16 Parks Road, Oxford OX1 3PH, UK. E-mail: jan.mol@materials.ox.ac.uk
bDepartment of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford OX1 3TA, UK
cQuantum Technology Centre, Physics Department, Lancaster University, LA1 4YB Lancaster, UK
First published on 10th July 2015
We demonstrate a robust graphene-molecule-graphene transistor architecture. We observe remarkably reproducible single electron charging, which we attribute to insensitivity of the molecular junction to the atomic configuration of the graphene electrodes. The stability of the graphene electrodes allow for high-bias transport spectroscopy and the observation of multiple redox states at room-temperature.
Carbon-based electrodes are appealing for contacting individual molecules.13,14 Unlike gold, which is the archetypical electrode materials for metal-molecule junctions, graphene has a low atomic mobility at room temperature, resulting in atomically stable electrodes.15 While different metals with a lower atomic mobility might also provide stable electrodes,16 the workfunction of these metals are typically not well matched to the discrete energy levels of the molecule as is the case for graphene.17 Furthermore, the two-dimensional nature of graphene results in weaker screening of a gate electric field compared to bulky three-dimensional electrodes, which means the distance between the gate electrode can be much larger than the distance between the source and drain electrodes whilst still maintaining the capability of gating the molecular orbitals. Here we demonstrate a robust graphene-molecule-graphene contacting geometry where a stable and reproducible single-molecule single-electron transistor (SET) architecture is achieved through careful design of the molecular building blocks and controlled formation of graphene nanogaps.
Modular molecular designs, consisting of a molecular backbone with specific side-groups for anchoring, spacing and self-alignment, in combination with graphene electrodes, have been proposed to overcome the variability issues that have long limited single-molecule electronics.7,18 Orbital gating of small molecules anchored to graphene electrodes has been demonstrated,15 but, to date, there are no studies of charge transport through complex modular molecules coupled to graphene electrodes. In this work, we study the charge transport through individual molecules in a graphene-molecule-graphene junction. The molecular wire, shown in Fig. 1a, consists of a zinc-porphyrin back-bone (black in Fig. 1a) with tetrabenzofluorene anchors (green in Fig. 1a). Porphyrin molecules provide a versatile platform for molecular device functionality,19 and have been widely investigated as such.20–22 Anchoring the molecular backbone to the graphene electrodes can be achieved either by covalent C–C bonding,23 or by π–π-stacking.15 The latter is especially of interest, as it leaves the electronic structure of the molecule largely unchanged, in contrast to thiol anchors which introduce gap-type states.24 Tetrabenzofluorene (TBF) ‘butterfly’ anchor groups used in this study are known to bind strongly to graphite surfaces25 and carbon nanotubes,26 and are robust in solvent solution.25 Density functional theory (DFT) calculations shown in Fig. 1b reveal that there is no steric hindrance to adsorption, and that the molecular wire relaxes across the graphene nanogap in a planar geometry. DFT calculations further indicate that the wavefunctions of the highest occupied molecular orbital (HOMO) are delocalised over the porphyrin backbone and anchor groups in contrast to the lowest unoccupied molecular orbital (LUMO) which are only localised over the porphyrin backbone, as shown in Fig. 1c. Overlap between the delocalised electron wavefunctions of the fully conjugated zinc-porphyrin system with the butterfly anchors allows for electron transport through the wire. The molecular backbone is separated from the butterfly anchor groups by a spacer (blue in Fig. 1a), which allows the anchor groups to bind to the defect-free graphene rather than to the graphene edges. In addition to the butterfly limpets, the molecule has two bulky side-groups (red in Fig. 1a). The side-groups make the molecular wire more soluble and prevent the central porphyrin from binding to the graphene electrodes.
We used lithographically patterned chemical vapour deposited (CVD) single-layer graphene,27,28 resulting in devices with greater reproducibility than those fabricated from few-layer graphene flakes.15 The graphene electrodes are fabricated using feedback-controlled electroburning28,29 and are typically separated by 1–2 nm. The chemical potential of the molecular wire is electrostatically tuned using the conducting silicon substrate as a back-gate (see Fig. 1b), which is separated from the molecule and graphene electrodes by a 300 nm thick silicon-oxide layer, resulting in a SET device geometry. The graphene electrodes are stable in air for at least several days. Molecules are deposited from a chloroform solution, after which the samples are immediately transferred into vacuum to prevent contamination. Fig. 1d shows typical current-voltage traces before (blue) and after (red) deposition of the molecule measured at 4 K. Before deposition of the molecule the current shows smooth exponential behaviour indicative of tunnelling through a single barrier. After deposition the presence of a molecule results in stepwise increases of the current as expected for sequential tunnelling through a double-barrier system. A scanning electron micrograph image of the device is shown in the inset of Fig. 1d.
First, we demonstrate reproducible single-electron transport through individual molecules. We show that the single electron charging is determined by the molecule rather than the microscopic details of the electrodes. Reproducible SET behaviour is measured in 10 out of 48 devices at 20 mK on which we deposited the molecular wire described above, as shown in Fig. 2. We find that for all devices Eadd = 0.37 ± 0.05 eV for the Coulomb diamond closest to equilibrium (zero gate voltage). The device statistics presented in Table 1 indicate that the measured SET behaviour of the devices shown in Fig. 2 arises from charge transport through approximately identical single-molecule transistors. In a control experiment using the same molecular backbone but without the TBF limpets (see Fig. SI2†), 20 devices were tested and no Coulomb diamonds were observed. From the reproducibility and from the control experiment we deduce: (i) molecules attach to the electrodes only when they are functionalised with anchor groups; (ii) the SET behaviour can be attributed to a molecule bridging the gap; (iii) the SET behaviour cannot be attributed to multiple molecules or to random carbon islands. The presence of multiple molecules would lead to multiple overlapping Coulomb diamonds whereas carbon islands would be expected to give more variable energy spacing Eadd. The observation of a constant energy spacing of Eadd ≈ 0.37 V for 10 out of 12 of the devices displaying Coulomb diamonds is a clear indication that there is only one active molecule in each device.
E add < 0.1 | E add ≈ 0.37 | No CB | |
---|---|---|---|
TBF anchors | 2 | 10 | 36 |
No anchors | 0 | 0 | 20 |
A residual degree of variability is still present in the molecular devices. The horizontal axes in Fig. 2 are scaled by an effective lever arm α which is a measure of the capacitive coupling between the gate and the molecule, and differs from device to device, with α = 0.006 − 0.04 estimated from the slopes of the Coulomb diamonds. The gate coupling observed in our devices with a 300 nm thick oxide are comparable to those reported for metal junctions on an oxide with a thickness of 40 nm.30 The small values of α indicate that the total capacitance is dominated by the source and drain electrodes, and is consistent with electrostatic calculations (ESI.II.C†). The variation in α can be attributed to differences in screening of the gate-field by the source and drain electrodes. The gate voltage to align the electrochemical potential of the electrodes with the Dirac point is greater than 40 V, thus giving an upper limit to the shift in the electrochemical potential of the electrodes as less than half the change in the potential of the molecule deduced from the slope of the Coulomb diamonds (ESI.II.D†). Trap states in the form of defects in the gate-oxide that can capture an electron and adsorbants on the graphene electrodes give rise to shifted and non-closing Coulomb diamonds (ESI.II.E†). Finally, we observe a significant variation in the current through the single-molecule devices, which can be attributed to differences in overlap between the anchor-groups and the graphene electrodes.
By looking more accurately at the transport spectroscopy of device 8, we can obtain the level spacing of the molecular orbitals and electron–electron interactions in the molecule. The stability of our molecular system (Fig. 3a) allows us to measure the energy spacing Eadd(N) between the ground state (GS) transitions from redox state N to redox state N + 1 of the molecule, from the height of the Coulomb diamonds. In the constant interaction model the addition energy consists of two parts:31 (i) the charging energy EC, due to the Coulomb interactions among electrons in the molecule and between electrons in the molecule and those in the environment; and (ii) the gap ΔHL between the HOMO and LUMO energy-levels. We can estimate the contribution of ΔHL and EC to the addition energy by comparing Eadd(N) for successive redox states and considering the spin-degeneracy of the molecular orbitals. We find that ΔHL = 0.05 eV for the N − 2 redox state and ΔHL = 0.06 eV for the N redox state. Several redox states have been observed in previous work on OPV molecules in gold nanogaps.4 The interpretation of the different contributions to Eadd can be further substantiated by comparing ΔHL with the single-particle energy level spacing which can be determined from the excited state spectrum for each redox state (see Fig. 3c). The stability of graphene allows us to extend measurements to bias-voltages beyond the limit set by electromigration for gold electrodes.4 We find that the first excited state of the N − 2 redox state aligns closely with the ground state of the N − 1 and N redox states. Likewise, the second excited state of N − 2 redox state aligns with the first excited state of N − 1 and N and the ground state of the N + 1 and N + 2 redox states. The single-electron energy spectrum seems to be largely independent of the number of electrons, with intervals dominated by the HOMO–LUMO energy separation. Renormalisation corrections of ∼3–4 eV have been observed experimentally and predicted theoretically for molecules in nanogaps32 and for molecules on graphite surfaces.33 For unscreened gas phase molecules our calculations yield an addition energy for one electron Eadd = 3.84 eV. From a simple screening potential (see ESI.III†) we estimate the reduction of the addition energy to be of the order of 3 eV, which is in reasonable agreement with our experimental findings.
Finally, we discuss the room temperature operation of the graphene-molecule-graphene transistors. Fig. 4 shows the stability diagram of device 2 measured at room temperature. Two Coulomb diamonds can be fully resolved, allowing us to probe the charge state transitions between three successive redox states. Using the same methodology as describe above we can estimate the charging energy EC = 0.28 ± 0.05 eV and HOMO–LUMO gap ΔHL = 0.09 ± 0.05 eV by comparing Eadd of the N and N + 1 redox states measured at room temperature.
In conclusion, we have demonstrated room-temperature charge- and energy-quantization in a reproducible graphene-molecule-graphene device geometry. The modular design of the molecular wire makes this approach applicable to a wide variety of molecular backbones. Specifically, the π–π anchoring of the molecule to the highly stable graphene nano-electrodes allows high-bias energy spectroscopy of the excited states and removes the need for statistical analysis of ensemble measurements. Our findings offer a route to a vast number of quantum transport experiments that are well established for semiconductor quantum dots, but at an energy-scale larger than kT at room temperature.
An approach that combines single molecules with novel two-dimensional materials and semiconductor fabrication technologies forms an attractive platform with which to realise scalable room-temperature single-electron transistor networks. Such an architecture could consist of individual molecules coupled to each other via graphene leads, with nearby graphene gate-electrodes to tune the orbital energy levels of the individual molecules. The gate-electrodes could be separated from the molecules by a two-dimensional insulator, to enable strong capacitive coupling between the gate and the molecule and allow the single-molecule transistors to exhibit gain. Here we have demonstrated the first step towards such an architecture: a reproducible single-molecule transistor. Further improvements in the graphene nanogap fabrication need to be made to reduce the offset charges and eliminate variability in the gate coupling as discussed above, providing a basis for the development of single-molecule electronics and also applicable to the fabrication of single-molecule based sensors and spin-based quantum computation.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/C5NR03294F |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2015 |