The first ultra-clean single-electron transistor based on solution-based synthesis of graphene nanoribbons

On February 2, 2023, the team of Professor Feng Xinliang of the Max Planck Institute for Microstructures and Physics in Germany, the team of Professor Feng Xinliang of the Technical University of Dresden, the team of Professor Yiyong Mai of Shanghai Jiao Tong University and the team of Professor Lapo Bogani of the University of Oxford published an article entitled “Exceptionally clean single-electron transistors from solutions of” in the journal Nature Materials molecular graphene nanoribbons”.

This achievement realizes the preparation of ultra-clean single-electron transistors by using graphene nanoribbons with excellent liquid phase dispersion, which provides a possibility for further study of the spin state and topological state of liquid graphene nanoribbons. In addition, because the construction method of this ultra-clean single-electron transistor is simple and efficient, the research threshold of quantum electronics is greatly reduced.

The corresponding authors of the paper are Feng Xinliang and Lapo Bogani; The first authors are Niu Wenhui, Simen Sopp, Alessandro Lodi and Alex Gee.

Graphene nanoribbons (GNRs) are a class of striped graphene with nanoscale width and length-to-diameter ratio greater than 10. Affected by the one-dimensional quantum confinement effect, the physical properties of GNRs such as photoelectricity, such as band gap and carrier mobility, are strongly dependent on their structure, including width, edge structure, heteroatom doping and other factors. Therefore, through precise regulation of their structure, the photoelectric properties and intrinsic quantum phenomena of the nanoribbons can be controlled, and spin-coherence times of up to microseconds can be achieved. However, only single-electron devices with high cleanliness can detect all electronic states of GNRs and apply them to quantum research experiments. This requires that only the spin and topological phenomena of a single GNR be detected during quantum electron transport.

In this work, Feng Xinliang’s team, Mai Yiyong’s team and Lapo Bogani’s team used side chains to modify graphene nanoribbons with rigid large groups to achieve liquid-phase single root dispersion of ultra-long nanoribbons. With the excellent liquid phase dispersion and long length of the nanoribbons, the team successfully fabricated single-electron transistors with periodic and clear coulombic diamond-shaped edges. It is worth emphasizing that this transistor exhibits strong electron-oscillator coupling. In addition, the construction method of ultra-clean single-electron transistors is very simple and efficient, which provides the possibility for further study of the spin state and topological state of liquid graphene nanoribbons, and greatly reduces the research threshold of quantum electronics.

Figure 1: Structure and preparation of alkyl chain modified graphene nanoribbons (GNR 1a, 1b) and rigid large group modified graphene nanoribbons (GNR 2).

To compare the relevant photophysical properties and transistor performance, the team prepared alkyl chain-modified GNR 1a and GNR 1b, respectively, and rigid large group-modified GNR2 (Figure 1). Thanks to the steric hindrance of rigid large groups, the π-π action of GNR2 is greatly weakened, making it exhibit excellent dispersion different from GNR1 in common organic solvents. By comparing the spectra of GNR 1b and GNR 2 above, the influence of dispersion differences on the photophysical properties of GNRs can be known. Compared to GNR 1b, GNR2 exhibits a high-resolution UV absorption peak with a maximum absorption peak of 544 nm accompanied by two shoulder peaks of 500 nm and 469 nm (Figure 2a). Compared with GNR 1b(λmax~557 nm), GNR2 exhibits obvious blueshift of absorption peak, which proves that the introduction of rigid large groups can effectively alleviate the π-π accumulation of nanoribbons. The UV absorption edges of GNR2 and GNR1b were tested at 617 nm and 660 nm, respectively, according to the formula:

The optical bandgap can be estimated to be 2.01 eV and 1.88 eV, respectively.

In addition, unlike the fluorescence spectrum in which GNR 1b is quenched by aggregation, GNR 2 exhibits a distinct fluorescence spectral signal, further demonstrating that the introduction of rigid large groups greatly improves the π-π accumulation of nanoribbons. By testing fluorescence emission spectra at different concentrations of GNR2, it was found that its fluorescence signal peaks exhibited strong concentration-dependent (Figure 2c). When the nanoribbon concentration is lower than 0.1 g L-1, two fluorescence signal peaks can be observed in the range of 600-650 nm, and the intensity increases with the increase of concentration, and reaches saturation when it reaches 0.1 g L-1. Above this concentration, the intensity of the two fluorescent signal peaks gradually decreases (Figure 2d), indicating that the aggregation of GNR2 becomes more severe as the concentration increases after the critical concentration is exceeded. In addition, AFM evidence suggests that GNR 1a exhibits nanoscale aggregation on the surface; Ultra-long GNR2 modified with rigid large groups exhibit significant “unbeaming”, presenting a single linear structure on the surface (Figure 2e).

Figure 2: Comparison of photophysical properties and AFM characterization of two nanoribbons.

Under the premise of realizing the single dispersion of ultra-long nanoribbons, Feng Xinliang’s team and Mai Yiyong’s team collaborated with the Lapo Bogani team of the University of Oxford to prepare single-electron transistors to further explore the quantum properties of nanoribbons. A single-electron transistor (SET) is a sensitive electronic device based on the Coulomb blocking effect. In this device, electrons flow through a tunnel junction between source-drain electrodes to a quantum dot (conductive island) between two tunnel junctions. The potential of the island can be adjusted by the gate. In the set up SET device (Figure 3a), the authors prepared graphene electrodes on nitrogen-doped silicon wafers with SiO2 layers by nanolithography and deposited GNRs onto the substrate by drop-coating, and the gap width between the two graphene electrodes spanned by GNRs was d = 1-10 nm (Figure 3b). Based on this device, the authors manipulated the device with the source-drain voltage VSD and gate voltage VG, while measuring the source-drain current ISD. In SET, electrons are transported through a single channel, and their potential can be adjusted by VG so that electrons tunnel into GNRs. Due to coulomb repulsion, the presence of one electron in the conduction channel obstructs the passage of other electrons, and SET can show regions where conductance is suppressed.

Figure 3c shows a stability plot of a SET built on GNR 1a. From the figure, you can see that the area of conductivity suppression is dark blue. A fuzzy coulomb diamond can be observed in the stability plot; But the diamonds are overlapping and have different sizes, without regularity. This suggests that due to the severe aggregation of nanoribbons, a “nanoribbon bundle” consisting of multiple nanoribbons crosses nanoscale channels and acts as conductive channels. Treating the additional energy Ea as rectangular quantum dots allows a rough estimate of the length l that the nanoribbon actually contributes. After calculation, the actual contribution length of GNR 1a is ~28, 33, 40, 50, 60, 90, 108 nm, and the existence of multiple values once again proves that GNR 1a acting as a conductive channel is actually a heterogeneous aggregate state. It is worth noting that for SET devices made of GNR 1a, there are no recognizable excited states in the Coulomb diamond.

In stark contrast, SET devices based on GNR2 exhibit ultra-pure conductive properties (Figure 3d). From the stability plot of SET, it can be seen that the coulomb diamond shape is similar in size and has obvious periodicity and well-defined edges, indicating that the device has a significant single-electron transport channel. Moreover, it can be judged from the bright yellow line parallel to the Kuhlomb-shaped boundary line that GNR 2 has a recognizable excited state in SET.

Figure 3: Quantum transport characterization of a single-electron transistor.

The ultra-pure transport properties in SET allow further observation of the details of the electron transport process. By amplifying a coulomb diamond, current suppression at certain coulomb edges can be observed at finer energy scales (Figure 4A). These excited states are always spaced 7 meV apart over the entire VG range, which coincides with the theoretical results (Figure 4a, e, f). Some of these inhibitory features appear only once every four coulomb rhomboids, which may come from spin-induced dark states, while most states are suppressed in each coulomb rhombus. As the temperature increases, the suppression of edge conductance is gradually lifted (Figure 4c). Similar to Frank-Condon’s principle in molecular spectroscopy, electron movement is most likely to occur with minimal changes in the position of the nucleus. Therefore, electroacoustic coupling γ suppress the transmission of low-level vibration states. A large number of characterizations demonstrated that the electroacoustic coupling strength in this nanoribbon single-electron device γ = 1.5 ± 0.2, indicating strong electron-oscillator coupling in the nanoribbon (Figure 4d). The results demonstrate the potential application value of liquid single-dispersed ultra-long graphene nanoribbons in quantum electronics.

Figure 4: Electron-oscillator coupling of nanoribbons.

(Source: Science Network)

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