Recently, the extraordinary properties of graphene such as high carrier mobility and thermal conductivity have made it a fascinating candidate for improvement of electron transport in nanoelectronics devices (Sharma et al. 2021; Shao et al. 2020; Jenkins et al. 2022; Yudhistira et al. 2015). Graphene nanoribbons (GNRs) can be produced by either cutting mechanically exfoliated graphene or by patterning epitaxial growth and show unique electrical characteristics (Wei et al. 2021; Bradford et al. 2023; Luo et al. 2022). Compared to other quasi-one-dimensional nanostructures like quantum wires, the electrical characteristics of GNRs are more reliant on their shape, size, and edge chirality (Gu et al. 2022). The electronic structure and transport features of GNRs are often investigated using the basic tight-binding model (Wong et al. 2019). It is observed that the characteristics of GNRs vary from metallic to semiconducting based on the size and configuration of their widths. The ZNRs are intrinsically metallic (Zhang et al. 2020), whereas, the nature of AGNRs can vary between metallic to semiconducting depending upon the number of dimer lines (N) involved in their width. For N = 3p and 3p + 1 (p = 1,2,3,..), the AGNR exhibits semiconducting behaviour, and for N = 3p + 2 or 3p-1, it is metallic (Lam et al. 2012). Devices based on GNRs, which may be produced via lithography on a graphene sheet, show significant technological impact and might take over silicon-based devices in future endeavors (Shen et al. 2015; Saraswat et al. 2021; Chuan et al. 2023).
The ability to cut graphene into a wide variety of forms and sizes has recently been shown by several groups, which opens the way to the creation of graphene-based nanodevices including graphene quantum dots, field effect transistors, quantum rings, and nano-circuits (Wang et al. 2017; Radsar et al. 2021; Jiang et al. 2019). In nano-circuits, the junctions play a pivotal role in deciding the transport properties of the graphene-based nanodevices. Several works have been reported by considering different junction configurations such as Motta et al. investigated transport properties of AGNR junction connected between two graphene electrodes using the first principle of quantum transport (Motta et al. 2012). They found that the transmission gap increases with the increase in the channel length. Jimenez et al. studied the I-V characteristics of AGNR where a channel with electrodes attached on both sides acts as a potential barrier and the overall considered shape behaves as a graphene tunnel diode (Jimenez et al. 2014). Hossain et al. investigated the tunneling behavior of AGNR by introducing different topologies of antidote in the channel region (Hossain et al. 2021). Teong et al. studied the I-V characteristics of different graphene nanoribbon junctions like H, W, and S-shape (Teong et al. 2009). Such types of junction configurations open the door for the design of tunnel diodes which has potential implications in switching circuits. The Z-shaped junction introduced by Chen et al. observed that the confinement of the quasibound state at the junctions produces conductance peaks around the fermi energy (Chen et al. 2008). In addition to nanodevice applications, several other shapes have also been reported such as Z, and L-shape, to enhance the conductance of nanocircuit interconnects which is also equally important for the realization of efficient GNR based nanoelectronic circuits (Ning et al. 2011; Ye et al. 2015). Hence, the nature of the junctions in GNRs play a crucial role in the flow of electrons which ultimately affects the circuit performance. However, a systematic understanding of the conducting behavior of the symmetry and asymmetry junction configurations of GNR-based fundamental shapes by using a single GNR is awaited.
In this work, investigation for conductance and density of state (DOS) of different graphene nanochannels with I-, U-, and H-shaped layouts have been carried out using a python-based simulation tool “Kwant”. The channels are formed using either ZNR or AGNR which possess symmetric and asymmetric junction configurations. The transport properties were investigated as a function of nanochannel width (WC). It is observed that in all the shapes as WC increases the conductance enhances around the zero Fermi energy. We obtained unit conductance for WC = 8, 12, and 16 number of atoms keeping the length of the ZNR inverted I-shape as 60 Å. However, in the case of U- and H-shaped structures, for narrow widths (WC = 8 or 12 atoms), the scatterings at the junctions and edges lead not only the reduction but also fluctuation in conductance. Whereas, for WC = 16, the scattering effect reduces and produces unity conductance. In the case of AGNR-based U-shaped structure, although the channel width satisfied the metallic condition (3p + 2, p is an integer), i.e., WC = 23, 29, and 35 atoms, the conductance is zero. Interestingly, in AGNR-based H-shaped junction for WC = 35 atoms produces unit conductance. Also, we show the effect of asymmetry on conductance in ZNR and AGNR based H-shaped structures. The current study creates a systematic understanding of the behaviour of graphene nanochannels for different dimensions and design configurations which can be a prerequisite for the design of graphene based tunnel devices and nanocircuit applications.