Quantum interference enhances the performance of single-molecule transistors.

Quantum effects in nanoscale electronic devices promise to lead to new types of functionality not achievable using classical electronic components. However, quantum behaviour also presents an unresolved challenge facing electronics at the few-nanometre scale: resistive channels start leaking owing to quantum tunnelling. This affects the performance of nanoscale transistors, with direct source-drain tunnelling degrading switching ratios and subthreshold swings, and ultimately limiting operating frequency due to increased static power dissipation. The usual strategy to mitigate quantum effects has been to increase device complexity, but theory shows that if quantum effects can be exploited in molecular-scale electronics, this could provide a route to lower energy consumption and boost device performance. Here we demonstrate these effects experimentally, showing how the performance of molecular transistors is improved when the resistive channel contains two destructively interfering waves. We use a zinc-porphyrin coupled to graphene electrodes in a three-terminal transistor to demonstrate a >104 conductance-switching ratio, a subthreshold swing at the thermionic limit, a >7 kHz operating frequency and stability over >105 cycles. We fully map the anti-resonance interference features in conductance, reproduce the behaviour by density functional theory calculations and trace back the high performance to the coupling between molecular orbitals and graphene edge states. These results demonstrate how the quantum nature of electron transmission at the nanoscale can enhance, rather than degrade, device performance, and highlight directions for future development of miniaturized electronics.

Porphyrin-fused graphene nanoribbons.

Graphene nanoribbons (GNRs), nanometre-wide strips of graphene, are promising materials for fabricating electronic devices. Many GNRs have been reported, yet no scalable strategies are known for synthesizing GNRs with metal atoms and heteroaromatic units at precisely defined positions in the conjugated backbone, which would be valuable for tuning their optical, electronic and magnetic properties. Here we report the solution-phase synthesis of a porphyrin-fused graphene nanoribbon (PGNR). This PGNR has metalloporphyrins fused into a twisted fjord-edged GNR backbone; it consists of long chains (>100 nm), with a narrow optical bandgap (~1.0 eV) and high local charge mobility (>400 cm2 V-1 s-1 by terahertz spectroscopy). We use this PGNR to fabricate ambipolar field-effect transistors with appealing switching behaviour, and single-electron transistors displaying multiple Coulomb diamonds. These results open an avenue to π-extended nanostructures with engineerable electrical and magnetic properties by transposing the coordination chemistry of porphyrins into graphene nanoribbons.

Exceptionally clean single-electron transistors from solutions of molecular graphene nanoribbons.

Only single-electron transistors with a certain level of cleanliness, where all states can be properly accessed, can be used for quantum experiments. To reveal their exceptional properties, carbon nanomaterials need to be stripped down to a single element: graphene has been exfoliated into a single sheet, and carbon nanotubes can reveal their vibrational, spin and quantum coherence properties only after being suspended across trenches1-3. Molecular graphene nanoribbons4-6 now provide carbon nanostructures with single-atom precision but suffer from poor solubility, similar to carbon nanotubes. Here we demonstrate the massive enhancement of the solubility of graphene nanoribbons by edge functionalization, to yield ultra-clean transport devices with sharp single-electron features. Strong electron-vibron coupling leads to a prominent Franck-Condon blockade, and the atomic definition of the edges allows identifying the associated transverse bending mode. These results demonstrate how molecular graphene can yield exceptionally clean electronic devices directly from solution. The sharpness of the electronic features opens a path to the exploitation of spin and vibrational properties in atomically precise graphene nanostructures.

Dispersion stability and biocompatibility of four ligand-exchanged NaYF4: Yb, Er upconversion nanoparticles.

Surface modification to obtain high dispersion stability and biocompatibility is a key factor for bio-application of upconversion nanoparticles (UCNPs). A systematic study of UCNPs modified with four hydrophilic molecules separately, comparing their dispersion stability in biological buffers and cellular biocompatibility is reported here. The results show that carboxyl-functionalized UCNPs (modified by 3,4-dihydrocinnamic acid (DHCA) or poly(monoacryloxyethyl phosphate (MAEP)) with negative surface charge have superior even-distribution in biological buffers compared to amino-functionalized UCNPs (modified by (aminomethyl)phosphonic (AMPA) or (3-Aminopropyl)triethoxysilane (APTES)) with positive surface charge. Subsequent investigation of cellular interactions revealed high levels of non-targeted cellular uptake of the particles modified with either of the three small molecules (AMPA, APTES, DHCA) and high levels of cytotoxicity when used at high concentrations. The particles were seen to be trapped as particle-aggregates within the cellular cytoplasm, leading to reduced cell viability and cell proliferation, along with dysregulation of the cell cycle as assessed by DNA content measurements. The dramatically reduced proportion of cells in G1 phase and the slightly increased proportion in G2 phase indicates inhibition of M phase, and the appearance of sub-G1 phase reflects cell necrosis. In contrast, MAEP-modified UCNPs are bio-friendly with increased dispersion stability in biological buffers, are non-cytotoxic, with negligible levels of non-specific cellular uptake and no effect on the cell cycle at both low and high concentrations. MAEP-modified UCNPs were further functionalized with streptavidin for intracellular microtubule imaging, and showed clear cytoskeletal structures via their upconversion luminescence. STATEMENT OF SIGNIFICANCE: Upconversion nanoparticles (UCNP) are an exciting potential nanomaterial for bio-applications. Their anti-Stokes luminescence makes them especially attractive to be used as imaging probes and thermal therapeutic reagents. Surface modification is the key to achieving stable and compatible hydrophilic-UCNPs. However, the lack of criteria to assess molecular ligands used for ligand exchange of nanoparticles has hampered the development of surface modification, and further limits UCNP's bio-application. Herein, we report a systematic comparative study of modified-UCNPs with four distinct hydrophilic molecules, assessing each particles' colloidal stability in biological buffers and their cellular biocompatibility. The protocol established here can serve as a potential guide for the surface modification of UCNPs in bio-applications.

Nanoscale junctions for single molecule electronics fabricated using bilayer nanoimprint lithography combined with feedback controlled electromigration.

Nanoimprint lithography (NIL) is a fast, simple and high throughput technique that allows fabrication of structures with nanometre precision features at low cost. We present an advanced bilayer nanoimprint lithography approach to fabricate four terminal nanojunction devices for use in single molecule electronic studies. In the first part of this work, we demonstrate a NIL lift-off process using a bilayer resist technique that negates problems associated with metal side-wall tearing during lift-off. In addition to precise nanoscale feature replication, we show that it is possible to imprint micron-sized features while still maintaining a bilayer structure enabling an undercut resist structure to be formed. This is accomplished by choosing suitable imprint parameters as well as residual layer etching depth and development time. We then use a feedback controlled electromigration procedure, to produce room-temperature stable nanogap electrodes with sizes below 2 nm. This approach facilitates the integration of molecules in stable, solid-state molecular electronic devices as demonstrated by incorporating benzenethiol as molecular bridges between the electrodes and characterizing its electronics properties through current-voltage measurements. The observation of molecular transport signatures, showing current suppression in the I-V behaviour at low voltage, which is then lifted at high voltage, signifying on- and off-resonant transport through molecular levels as a function of voltage, is confirmed in repeated I-V sweeps. The large conductance, symmetry of the I-V sweep and small value of the voltage minimum in transition voltage spectroscopy indicates the bridging of the two benzenethiol molecules is by π-stacking.