Spin Transport Properties in 1D DNA and Electrically Doped Iron Quantum Dot Organo-metallic Junction: A First Principle Paradigm

: The DNA sensor has emerged as strong candidates for next generation ultra low power application due to its self assemble technique. Non- Equilibrium Green’s Function (NEGF) along with Density Functional Theory (DFT) based First Principle approach is used in investigation of spin transport properties along with quantum scattering transmission characteristics of DNA sensor via Iron (Fe) quantum dots (QD) electrodes at room temperature. Electrically doped Fe QD plays an important role in spin transport mechanism. This electrical doping concentration and weak coupling strength between DNA and Fe QD organo-metallic junction effect into the tunneling contact resistance (TCR) along with quantum-ballistic transmission and junction conductivity of parallel and anti-parallel configuration of this analytical model representation. It has been observed that higher current has been achieved for parallel configuration when compare with anti-parallel configuration at same bias voltage. This voltage-current characteristic is significantly modulated due to the electrical doping effect. This spin transport property shows that this system can well perform for anti parallel configuration. High tunnel organo-metallic resistance approximately 99.9% is observed even at 0V bias voltage. TOMCR remains large at upper bias voltage


Introduction
Spintronics which unlike of electronics uses spin plays a crucial role in the era of nanotechnology.
Intrinsic spin of the electron along with its magnetic moment in addition to its fundamental electronic charge is the basics of Spintronics. It is considered to be as one of the most significant rising research arenas with enormous possibilities to provide low power consumption, high speed, and high density logic which is applicable for memory electronic devices. This is also useful for lower threshold current and high-power lasers which are applicable for optoelectronic devices as a basis for circularly polarized light.
It is not likely that always applicable for magnetic tunnel junction but also applicable for semi-conductor tunnel junction [1,2]. The effective spin diffusion length is directly proportional to the decay rate of spin current, along with the spin accumulation and the spin chemical potential along the spin current flow direction. In the case of semiconductors the effective spin diffusion length depends on the charge current flowing parallel to the spin current. Spin current is the key concept for Spintronics. There are two significant configurations are described considering spin transport phenomenon. Parallel configuration (PC) and anti-parallel configuration (APC) are the two major configurations which are associated with this spin transmission. These tunnel junctions can be made either for magnetic tunnel or semi-conductor tunnel junction. [1][2][3][4][5]. For the fabrication of Magneto-Tunnel Junctions (MTJ), generally very thin metal oxide layers are used for example SiO2, MgO, HfO2, ZrO2 etc. These materials are used as tunnel junctions. Due to limited performance of these materials, inter-diffusion and metal oxide defects affect into these fabrication of tunnel junction devices [6][7]. In some articles, it is illustrated that, Fe doped carbon nano tube (CNT) is used as tunnel junction device. Boron nitride nanotube is also used for this type of spin transport study [3,4]. Some of the researchers are also advised that SiCNT can be a substitute to CNT due to their improved performance for its large strength, high density, excellent thermal shock resistance property, high operating frequency performance, against harsh environment etc.
These properties make this material useful for further studies [8]. Protonation controlled spin transport properties are also conducted for magnetic single molecule junctions. Protonation plays catalytic role in this type of spin transport phenomenon [9]. Multiple molecular effect for example quantum transport, spin-filtering etc. can be investigated for single molecule magnet like Mn(dmit)2 and phosphorene electrodes using first principle approach [10]. This spin transport phenomenon can be well explained using first principle approach both for organic and in-organic tunnel junctions [11][12][13].
In this paper, we investigate spin transport properties for DNA and Fe QD organo-metalic junction at room temperature. Two configurations PC and APC for this type of junction are investigated using DFT and NEGF based first principle formalisms. Spin dependent quantum ballistic transmission is investigated in this paper. A comparative study shows the novelty of this tunnel junction in Table 1

Materials and Method
This paper presents organo-metallic junction which is formed using DNA and Fe-QD. This Fe-QDs are also used as electrodes. Though weak coupling exists between DNA and two electrodes but it does not affect spin transport phenomenon. This quantum-ballistic model is analytically simulated using Atomistix Tool kit-Virtual Nano Laboratory (ATK-VNL) software simulation package version 13.8.0 [14]. This software simulation is used for self-consistent calculations along with Spin polarized Generalized Gradient Approximation (SGGA) exchange correlation procedure. The relaxed geometrical structure with minimum stress is obtained using Quasi-Newton geometry relaxation method. The interatomic force and stress is kept minimum. The inter-atomic force is 0.01eV/Å and stress is also 0.01eV/Å3.
These reduced force and stress are considered to be optimized to get maximum spin transport current flow. This simulation method comprises with single zeta polarization method. The cut-off energy is kept 75Ry with 1×1×100 k-point sampling along x, y, z direction respectively. Electrode temperature is kept 300k to get faster convergence. For PC, the relative spin is to be set at 1/0/1 and for APC it is set to be 1/0/-1 [1]. The gap between electrode and DNA is kept minimal to achieve maximum transport current.
The various simulation parameters are shown in Table 2.

Result and Discussion
This paper shows an organo-metallic junction which is created using DNA and Fe QD. The two ends of this atomistic model are two QDs. The weak coulomb interaction is held between DNA and electrodes.
These electrodes are electrically doped to provide faster and highly impressive spin transport quantumballistic current flow through the central molecular region. The front view of this organo-metallic junction is shown in Fig. 1   These figures show that due to the quantum back scattering effect the device has been affected, hence the transmission peaks are slightly displaced from the bias window (-2.0V to +2.0V) for some of the cases which is shown in Fig. 3. It has been observed that transmission peaks as well as Highest Occupied Molecular Orbital -Lowest Un-occupied Molecular Orbital (HOMO-LUMO) gap significantly depends on electrical doping concentration. For APC configuration, we observed maximum gap between HOMO-LUMO during spin up and spin down transport conditions. If the HOMO-LUMO gap is maximum, it signifies high thermodynamic stability which was explained by Jahn -Teller effect [16,17]. But in the same time, it will reduce channel conductivity as the barrier height is more. Thus spin transport current for this APC is lower. Whereas for PC HOMO-LUMO gap is much lower when compared to APC. Thus it provides both thermo dynamic stability as well as higher channel conductivity due to lower barrier height. Therefore, more spin transport current can be observed for PC. High transmission peaks depict higher number of available quantum scattering channels. Therefore, it is also observed that for PC, large V-I characteristics for PC and APC are shown in Fig.4 and Fig. 5 respectively. It has been observed that due to lower barrier transmission height, channel conductivity is high, hence large amount of current flows through the central molecular region for PC. High current is observed both for spin up and spin down configuration for PC. Whereas due to large barrier height, low channel current is observed for APC both for spin up and spin down configuration. In case of PC, spin up current is much more compared to spin down current. It can be observed from transmission spectra plot and spin current graphs, that though higher transmission peaks are available for spin down condition but due to smaller barrier height large current is available for spin up condition for PC. In this Eq. (4), IPC and IAPC are the sum of spin current (spin up and down) for PC and APC respectively for this bio-system. At 0V bias voltage, current is almost 0, therefore, TOMCR is calculated as 99.99% which is even increased to 99.9% and remains hold its constant value upto 0.4V bias voltage where IAPC is almost 0. TOMCR value is almost remains high upto 0.4V and then decreases due to the slight increased values in spin current for APC. TOMCR with respect to bias voltage is plotted in Fig. 6. This graph shows that almost constant line has been achieved due to the lower values of spin current of APC.
Spin injection factor or spin current efficiency η is calculated using Eq. (5).
Spin current efficiency is plotted with respect to bias voltage and shown in Fig. 6. Spin current is almost negligible and considered as 0 at 0V bias voltage; therefore η has calculated using conductance at equilibrium. For PC η is obtained almost 100% at all bias voltage. Thus spin -filtration which is approximately 100% , has been obtained for this bio-molecular device for all bias voltage. It is observed from Fig. 7 that 100% spin filtration is also obtained for APC. This is due to the less difference between spin up and spin down current are obtained during APC. This leads to give almost 100% spin-filtering effect. The cross-tick table shows the novelty of this device when compared with existing spin dependent devices in Table 3.