TEMPLATE DESIGN © SIMULATION OF RESONANT TUNNELING DIODES FOR NANOELECTRONIC APPLICATIONS Palla Pavankumar, Perumalla V. N. Pradeep Kumar, and Reddy Bhanu Prakash Department of Electronics and Communication Engineering, SRM University, Kattankulathur, Chennai, Tamil Nadu , India. Guided by: Dr. P. Aruna Priya. ABSTRACT Molecular electronics is the utilisation of a molecule or a group of molecules as an electronic device in a circuit. Experiments have shown how such devices could be fabricated with useful properties such as rectification, hysteresis and negative differential resistance (NDR). In this paper we investigate the transport behaviour of the molecular resonant tunneling diode (RTD) addressed by gold electrodes from both sides. A sulphur atom at each joins the molecular resonant tunneling diode to the bulk Au (111) electrode. The molecule with the discrete energy levels is connected by two semi-infinite electrodes with the continuous band. To describe the interaction correctly at the interface between the sulphur atoms and the gold electrodes, we treat an extended molecule exactly by including one gold atom on each side. RTD exhibits a negative differential resistance (NDR) region in its current-voltage characteristics which can be exploited for high speed and low power circuits. Here we design a new intramolecular complex with donor and acceptor molecular subunits to explore the possibility of its working as an RTD. We calculate the I-V behaviours using a self consistent method and calculate the current and the differential conductance.  The molecule which consists of appropriate donor and acceptor, if placed between suitable electrode contacts, forms a diode. That is, electron flow through the structure should be strongly preferred only in one direction.  One fundamental step in molecular electronics is to study the electronic properties of functional molecules and explore potential molecules for device implementation.  The negative differential resistance characteristic of these devices, achieved due to resonant tunneling, is also ideally suited for the design of highly compact, self-latching logic circuits.  we are going to design a new intramolecular complex with donor and acceptor molecular subunits and to show how it works as a RTD.  We have substituted –NO 2 group which acts as acceptor and –NH 2 as donor in the benzene ring. This introduces a redox centre in the middle benzene ring.  The electron withdrawing nitro group is responsible for NDR behaviour, whereas the electron-donating amine group gives rise to a bound state in the molecule. Fig.1: Structure of molecular RTD INTRODUCTION FORMALISM AND COMPUTATIONAL SCHEME  The gold electrodes are attached at both sides and a sulphur atom at each end joins the molecular diode to the bulk Au (111) electrode. Fig 2. Molecule attached with gold electrodes.  An electron incident from the source with energy E has a probability T (E) of being transmitted through the molecule to the drain. By calculating this transmission probability for a range of energies around the Fermi function E f of the lead, current is calculated using the Landauer formula. T (E) = trace (Γ 1 G Γ 2 G + ) Electrochemical potential µ 1, 2 = E ± Broadening Γ 1, 2 = i (∑ 1, 2 -∑ 1, 2 + ).  The self energy functions ∑ 1, 2 are used to describe the effect of contacts on the device. The molecular Green’s function G [9] [10] is given by G (E) = (ES – H + U SCF - ∑ 1 - ∑ 2 ) -1  The device is described by a Hamiltonian matrix H and overlap matrix S. The self consistent potential is given by U SCF =U (N-N eq )  U is the charging energy; N-N eq is the change in the number of electrons from the equilibrium value N eq.  Optimising the geometry of the molecule is the most time consuming process. The time taken depends upon the number of atoms in the molecule, may be 4 to 10 days.  Calculating the Equilibrium properties of the molecule is the time consuming process of the simulation. For PDT molecule it takes minutes to build the Dos_TE.mat file. It may take up to 1-2 hours for bigger molecules.  The contacts are assumed to have a constant density of states in the energy range of interest.  Structural changes in the molecule under bias are not considered. RESULTS AND DISCUSSION  Our calculations are based on the Huckel-IV codes and it is assumed that the molecule under investigation forms symmetric contact with two semi-infinite gold electrodes.  A resonant-tunneling diode is made by placing two insulating barriers in a molecule, creating between them an island or potential well where electrons can reside.  Whenever electrons are confined between two such closely spaced barriers, quantum mechanics restricts their energies to one of a finite number of discrete "quantized" levels. Fig 3. Current-Voltage characteristics of molecular Resonant tunneling diode. Fig 4. Conductance-Voltage characteristics of molecular Resonant tunneling diode.  When the energy of the incoming electrons aligns with that of one of the internal energy levels, the energy of the electrons outside the well is said to be "in resonance" with the allowed energy inside the well. Then, maximum current flows through the device at this resonant voltage or peak voltage (V p ) called the peak current (I p ).  The measured peak-to-valley ratio is approximately 1.33: 1. [1] M. A. Reed, “Molecular-scale electronics,” Proc. IEEE, vol. 87, no. 4, pp. 652–658, Apr [2] G. L. Fisher, A. E. Hooper, R. L. Opila, D. L. Allara, and N. Winograd, “The interaction of vapor-deposited Al atoms with COOH groups at the surface of a self-assembled alkanethiolate monolayer on gold”, J. Phys. Chem. B, vol. 104, pp. 3267–3273, [3]S.Datta, Quantum Transport :Atom to Transistor, Cambridge University Press, Cambridge, UK, REFERENCES REALISTIC CONSTRAINTS