Spin-State Switching of Manganese Porphyrin by ... - ACS Publications

Article pubs.acs.org/JPCC

Spin-State Switching of Manganese Porphyrin by Conformational Modification Dibyajyoti Ghosh,† Prakash Parida,§ and Swapan K. Pati*,‡ †

Chemistry and Physics of Materials Unit, and ‡Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India § Institute for Theoretical Physics, University of Regensburg, D-93040 Regensburg, Germany S Supporting Information *

ABSTRACT: Controlled spin-state switching in small molecules is of great interest for recent molecular spintronic and spin-caloritronic applications. The 3d transition metal incorporated porphyrin molecules with stable paramagnetic states are one of the most explored classes of molecules for this purpose where adsorption and desorption of small gaseous molecules (e.g., CO, NO, O2) on the transition metal center show efficient control over the spin states of metalloporphyrins. However, in the present study, using on-site Coulomb interaction incorporated density functional theory (DFT + U), we demonstrate reversible spin-state switching of NOadsorbed manganese porphyrin (MnP) on top of a gold (111) surface by inducing conformational change in the molecular geometry. In this approach, mechanical manipulation by a scanning tunneling microscope (STM) tip can reversibly interchange the binding mode of the Mn−NO bond between ground-state linear and metastable bent conformations. And this modification leads to spin-state switching between the low-spin state (S = 0) of the linear geometry and intermediate spin (S = 1) of the bent conformer. Further, nonequilibrium Green’s function based studies reveal that, in a two-terminal device architecture, spinpolarized electronic transport through this MnP-based molecular junction can efficiently be switched off/on upon the conformational change. Thermally induced current and thermopower can also be modified distinctly when we introduce temperature bias in this nanodevice. Interestingly, precise tuning of the Fermi level of the device results in generation of pure spin thermopower, which is highly demanding for potential spin-caloritronic application.

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INTRODUCTION Stable magnetic ordering and its controlled manipulation in low-dimensional systems is a rapidly emerging area for spintronic applications.1,2 Particularly, the strong possibility to fabricate much denser and faster nanocircuits using single magnetic molecules has created a huge interest in technological as well as fundamental research.2−4 A number of studies in these molecular spintronic systems have already shown several promising properties such as giant magnetoresistance,5 spin injection,6 spin filtration,7,8 spin current switching,9 negative differential resistance,10 memory effect,4 etc. More recently, these molecular junctions also have revealed their efficient thermoelectric properties where heat gets converted to electrical power.9,11−13 Effective coupling between spintronics and thermoelectrics in molecular devices opens up a highly promising, relatively unexplored field of researchmolecular spin caloritronics.9,14 Pioneering studies where, even in absence of electrical bias, spin-polarized current appears due to temperature difference depict the possible application of this spin caloritronics in the power-efficient energy-harvesting devices.14 Small isolated molecules with stable paramagnetic ordering are the perfect ingredient for the molecular spintronic as well as spin-caloritronic applications.2,15 In this regard, spin-bearing first-row transition metal (TM) centered metalloporphyrins © XXXX American Chemical Society

(TMP), having localized 3d states, are one of the hugely explored systems.16−21 Recent experimental as well as theoretical studies have already revealed a number of fascinating electronic and spintronic properties.16,18,19,22−26 Particularly, in spintronics, one of the major targets is to control the spin states of the magnetic material by various external influences such as light irradiation,27 temperature control,28 molecular adsorption,17,19 substrate interaction,18 etc. Recently, various studies demonstrate an efficient way to manipulate the spin state of metalloporphyrins (TMP) by reversibly attaching small gaseous molecules at the metal center.17,19,29 Generally, in this process, small gaseous molecules have to be adsorbed and thermally desorbed repeatedly to tune the spin state of the metalloporphyrins.17 As a result, the process becomes quite energy consuming and not suitable for practical purpose as the desorption step generally requires very high temperature.17,19 In the present study, first we perform a thorough investigation about the effects of NO binding on TM of TMP (TMs are V−Co). Further, we concentrate on exploring the conformation-induced spin-state manipulation of NOReceived: November 16, 2015 Revised: February 4, 2016

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DOI: 10.1021/acs.jpcc.5b11227 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

for spin-polarized systems, as implemented in the TranSIESTA package.42 The transmission function is formulated as follows:

ligated manganese porphyrin (MnP−NO), adsorbed on the (111) surface of gold. Initially, NO binds to the Mn atom in the ground-state linear conformation. However, the Mn−NO bond conformation can reversibly be modified to metastable bent Mn−NO geometry by laterally moving the closely placed gold scanning tunneling microscope (STM) tip in either constant tunneling current or constant tip height mode. Interestingly, along with this reversible conformational change between linear and bent conformations, one can efficiently switch off (S = 0) and on (S = 1) the spin moment of the MnP. Further, nonequilibrium Green’s function (NEGF)-based transmission calculations evidently demonstrate the prominent dependence of electron- and spin-transport characteristics upon the spin state of the MnP−NO-based molecular bridge. Furthermore, investigation about the thermoelectric properties of these nanojunctions reveals their potential usage in spin-caloritronic applications. Most importantly, controlled tuning of the Fermi level of the system eventually results in generation of pure spin thermopower without any charge component in this device.

T (E) = Tr[ΓL(E)G(E)ΓR (E)G†(E)]

(1)

where the G(E) is Green’s function and it is evaluated from the Hamiltonian of the central region and self-energies. ΓL/R(E) is (−2 times) the imaginary part of the self-energies of the left and right electrode. Further, we use the Landauer−Buttiker formula to calculate current:43 I=

⎛ G0 ⎞ ⎜ ⎟ ⎝ e ⎠

∫μ

μR L

[f (E , μL ) − f (E , μR )]T (E) dE

(2)

where μL and μR are the chemical potentials of the left and right electrode, respectively, and G0 is the quantum of conductance. Here, f(E, μL/R) represents the Fermi−Dirac distributions at the left/right electrodes.

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RESULTS AND DISCUSSION Binding of NO with 3d Metalloporphyrin. Geometry optimization without any constrains reveals that all 3d TM (V− Co) incorporated porphyrin molecules (i.e., TMP) are planar (see Figure S1 in the Supporting Information). Details of the structure and magnetic ground state of these TMP can be found in the Supporting Information. Note that two hydrogen atoms of oppositely positioned bridging carbon of the molecule have been substituted by ethynyl groups to make the metalloporphyrin rings suitable for electronic transport measurements (discussed in the Supporting Information). As shown in Figure 1 (MnP as an example; others are given in Figure S3 in the Supporting Information), upon exposure to

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COMPUTATIONAL DETAILS We perform spin-polarized density functional theory (DFT)based calculations as implemented in the Vienna ab Initio Simulation Package (VASP) for the study.30 The Perdew− Burke−Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) is used to include exchangecorrelation contributions.31 For all the calculations, projected augmented-wave (PAW) potentials are used with a plane-wave basis set having a cutoff of 400 eV.32,33 We carry out geometry optimizations for all the systems without imposing any symmetry constraints where interatomic forces are relaxed up to 0.01 eV/Å. Further, the total energy convergence is chosen as 10−8 eV for all static calculations. As only GGA functionals cannot describe partially occupied, strongly localized d orbital containing atoms accurately, we use the widely applied GGA + U method.34,35 Except for NO-adsorbed MnP molecules, we consider correlation energy (U) as 4 eV and exchange energy (J) as 1 eV, which are well-testified for metalloporphyrin complexes by several previous studies.17,18,21,36,37 However, as these parameters fail to reproduce experimental ground-state geometry of NO-adsorbed MnP,17,38 we reparametrize U and J as U = 2, J = 1 (details can be found in the Supporting Information) for this particular molecule. We also consider the Vosko−Wilk−Nusair modification for spin-polarized calculations to interpolate the correlation energy.39 Note that good agreement with available experimental data of most of these metalloporphyrin molecules strongly validates our overall computational approach for the present study (see Table 2 of the Supporting Information). To model the gold (111) surface, we consider three atomic layers of (8 × 8)Au where each layer contains 64 atoms. For all the surface-included calculations, we fix the geometry of surface Au atoms to reduce computational effort. Note that previous reports on adsorption of MnP on Au(111) with and without performing relaxation of the surface show a negligible difference in geometric, electronic, and magnetic properties.40 To sample the reciprocal space, we use 2 × 2 × 1 Monkhorst−Pack k-points. As standard PBE functionals-based DFT calculations cannot include the van der Waals interaction between MnP and the Au(111) surface properly, we apply the DFT-D2 method as suggested by Grimme and implemented in VASP.41 For electronic transport property calculations, we use the NEGF methodology extended

Figure 1. Optimized geometry of (a) MnP−NOlinear and (b) MnP− NObent. Side views of the structures are shown in the inset. White, black, blue, red, and green atoms represent H, C, N, O, and Mn, respectively.

gaseous NO, these metalloporphyrin molecules form a stable axial coordinate bond with the nitrogen of NO in either of two binding geometries, i.e., linear or bent (i.e., TMP−NOlinear or TMP−NObent). Importantly, the relative stability between these two binding modes of TM−NO perfectly agrees with an earlier prediction by Feltham and Enemark that correlates the total number of d electrons in TM and π* orbital electrons of NO to the TM−N−O angle.44 Metalloporphyrins of early transition metals, i.e., VP, CrP, and MnP, form almost perfect linear TM− N−O bonding (∠TM−N−O ≈ 180°), whereas CoP (i.e., late transition metal containing porphyrin) gets stabilized in proper B


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Figure 2. Partial charge densities of the (a) dπ−pπ* orbital of MnP−NOlinear, (b) 3dz2−pπ* orbital of CoP−NO, and (c) dπ−pπ* orbital and (d) 3dz2−pπ* of FeP−NO. The isosurface value is considered as 0.025 e/Å3 for all charge densities.

Figure 3. Partial density of states (pDOS) of (a) MnP−NOlinear and (b) MnP−NObent molecules. (c) dπ−pπ* orbital, (d) 3dz2−pπ* orbital, and (e) spin-density plot for MnP−NObent. The isosurface values are considered as 0.015 and 0.05 e/Å3 for charge and spin density, respectively.

nature, where the linearly bonded (i.e., ∠Fe−N−O ≈ 180°) FeP−NO is only 0.13 eV higher in energy than the groundstate bent geometry.45 Interestingly, the energy barrier between linear and bent conformers of MnP−NO also appears to be small. The ground state, i.e., MnP−NOlinear, remains energetically more stable by 34 meV than the intermediate bent geometry (∠Mn−N−O ≈ 143°). Thus, the bent conformation can be considered as a metastable equilibrium point in the potential energy surface with respect to bending distortion. Most importantly, the spin state of MnP, which shows low-spin configuration, i.e., S = 0, due to linear NO binding, switches on the spin moment, i.e., S = 1, as the binding mode changes to bent conformation. However, in the case of Fe, note that the experimentally46 as well as theoretically45 observed paramagnetic nature of FeP−NO (i.e., S = 1/2) remains unaffected with respect to the change in Fe−NO binding conformation. In the present study, as we are interested on the complete spin switching of metalloporphyrin by precisely controlling the TM−NO binding mode, MnP−NO appears to be the most

bent conformation (∠Co−N−O ≈ 122°). FeP shows an intermediate value of Fe−N−O bond angle, i.e., ≈143°. As V, Cr, and Mn atoms have low-lying singly occupied 3dπ orbitals (i.e., 3dxz and 3dyz), the orbital of NO forms dπ−pπ backbonding with these metal atoms, resulting in linear binding between TM and NO (see Figure 2a). However, for CoP, where metal 3dπ orbitals are completely filled, the nonbonded 3dz2 orbital only remains as a singly occupied molecular orbital. Thus, the pπ* orbital of NO creates in-phase σ-type bonding with the 3dz2 orbital of Co, resulting in a bent Co−NO bond in CoP−NObent (see Figure 2b). Now, as shown in Figure 2d, in the case of FeP, the pπ* orbital of NO weakly overlaps with the 3dz2 orbital of Fe and consequently forms a bonding molecular orbital where an unpaired electron gets accommodated. Furthermore, the pπ* orbitals of NO hybridize with partially occupied 3dπ orbitals of Fe as well (see Figure 2c). Thus, due to the involvement of both 3dz2 and 3dπ orbitals toward NO binding, an intermediate Fe−N−O bond angle (i.e., 143°) appears in this case. This Fe−N−O bond is also quite flexible in C


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appearance of a finite magnetic moment, arising from ≈2.5 spin 1/2 objects on the Mn atom. Furthermore, as minority-spin density gets localized on bent NO, the total magnetic moment of MnP−NObent reduces to 2 spin 1/2. Spin density in Figure 3e also clearly shows major localization of oppositely polarized spins over Mn and NO. Therefore, in its bent geometry, NO acts as σ-donor weak-field ligand and keeps 3d orbitals partially filled. Experimentally, upon conformational change in Mn−NO binding, XMCD-based spectroscopic study can prominently exhibit the reappearance of the characteristic spectral signature, indicating the stability of the magnetically active state of MnP− NObent. Effect of the Au(111) Surface. Generally, metalloporphyrin molecules get chemisorbed or physisorbed on the welldefined metallic surfaces (magnetic or nonmagnetic) and exhibit a number of substrate-assisted exciting properties.17,18,20 However, as the present study is majorly focused on the modulation of molecular electron and heat transport characteristics upon conformation-induced spin-state switching of MnP−NO, a weakly interacting surface should be a perfect choice. Thus, we consider a nonmagnetic Au(111) plane as supporting surface to deposit the MnP molecules. We find interaction between MnP and the Au surface, with or without adsorbed NO, appears to be quite site-unspecific, indicating characteristic weak surface−molecule overlap. Further, in the presence of Au surface states, the narrow molecular states of MnP−NO become broadened (see the molecular DOS in Figure 4b), which again indicates a weak molecule−surface

appropriate one. Note that, though V-porphyrin also shows prominent conformation-induced spin switching, we are excluding it from our discussion due to its poor stability in VP form.47 Thus, in the following sections, we focus on the microscopic reasons behind the spin switching of this molecule upon conformational change of the Mn−NO bond. Binding of NO with MnP. Binding energies of NO to MnP in its linear and bent conformations are found as −1.56 and −1.52 eV, respectively. This indicates strong coordination bond formation between Mn and N of NO. Moreover, the axial coordination causes minor structural modifications to the MnP molecule. Most notably, the Mn atom comes out of the porphyrin ring plane (defined by the four pyrrolic nitrogen atoms) by 0.32 Å (0.29 Å) for linear (bent) binding mode. This out-of-plane movement of metal upon uniaxial coordination can also be observed for other metalloporphyrin systems.36 Further, imposing the planarity (i.e., keeping the Mn atom inside the porphyrin core) on MnP−NOlinear (MnP−NObent), the molecule gets destabilized by an energy of 0.52 eV (0.32 eV). As the Mn−NO bond forms due to the significant 3dπ− pπ* (for linear conformer, Figure 3a and Figure 2a) as well as 3dz2−pπ* (for bent conformer, Figure 3b and Figure 3, parts c and d) orbital overlaps, the Mn atom lifts up from the core of the cavity to facilitate these hybridizations. The out-of-plane shift of Mn also helps the core of the porphyrin ring to reduce the in-plane electron accumulation, which arises due to the doubly (for linear conformer) or singly (for bent conformer) occupied nonbonding dxy orbital of Mn. Note that stronger 3dπ−pπ* orbital overlap and complete occupancy of the inplane dxy orbital for the linear binding mode results in higher out-of-plane shift of the Mn atom than the other one. When optimized structures of MnP−NOlinear and MnP− NObent are compared, it is evident that conformational modification only alters the Mn−NNO bond length and keeps the rest of the MnP geometry almost unchanged. This change in Mn−N(NO) bond length appears due to the weakening of 3dπ−pπ* back-bonding in the bent conformation. Detailed discussion can be found in the Supporting Information. Importantly, this conformational modification in Mn−NO binding mode efficiently switches off/on the molecular spin moment. In Figure 3a, partial density of states (pDOS) for 3d orbitals of Mn and 2p orbitals of NO of MnP−NOlinear depict that spin-degenerate 3dπ orbitals of Mn overlap with π* orbitals of NO and form spin-unpolarized dπ−pπ* back-bonded molecular states. Further, pDOS for 3d orbitals of Mn in MnP (Figure S4 in the Supporting Information) and MnP− NOlinear (Figure 3a) evidently demonstrates that, upon axial ligation to linear NO, 3dz2 and 3dx2−y2 orbitals shift to the higher energy region due to strong crystal field splitting. This alteration in 3d orbital ordering results in increased electron population in nonbonding 3dxy and bonded 3dπ−pπ orbitals; however, it makes the 3dz2 and 3dx2−y2 orbitals completely empty. Thus, in the linear binding mode, π-acid ligand NO acts as a strong-field ligand and paired up all initially unpaired electrons, exhibiting a spin crossover from high-spin state (S = 5/2) to low-spin state (S = 0). This quenching of spin moment in MnP upon exposing it to gaseous NO has already been demonstrated experimentally by observing the absence of X-ray magnetic circular dichroism (XMCD) signals.17 On the other hand, in bent conformation, majority-spin 3dz2 and minorityspin 3dπ orbitals of Mn mainly overlap with π* orbitals of NO (Figure 3b), whereas electrons from majority-spin 3dπ and 3dxy orbitals remain almost inert to the NO binding and result in the

Figure 4. (a) Side view of optimized geometry of MnP−NOlinear on top of the Au(111) surface. Right-top inset shows the top view of the structure. (b) Molecular DOS for isolated MnP−NObent and at on top of the Au(111) surface. (c) Spin-density plot for MnP−NObent on top of Au(111). Isosurface is considered as 0.05 e/Å3.

coupling, without formation of any strong chemical bonds. Detailed analysis about surface−MnP interaction can be found in the Supporting Information. Most importantly, conformation-induced spin-state switching of MnP−NO remains unchanged even in the presence of the Au surface. After adsorbing to the Au surface, MnP−NOlinear and MnP−NObent still remain in the spin state of S = 0 and S = 1, respectively. Further, comparing Figures 3e and 4c, we find a negligible effect of the Au(111) surface over the spin density of MnP− NObent. Thus, we can safely exclude the presence of this inert adsorbent for further calculations. STM Tip Induced Conformational Change. To reversibly manipulate the conformation of MnP−NO between linear and bent form, we propose the use of an atomically sharp nonmagnetic gold STM tip.48 The STM tip is widely used to precisely modify the conformation as well as magnetic, electronic, and mechanical properties at a single-molecule level.22,25,48 Particularly, lateral motion of the tip along a D


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Figure 5. Optimized geometry of (a) MnP−NOlinear−Au4 and (b) MnP−NObent−Au4. Side view of the structures are shown in the inset.

interaction between the tip and NO becomes much stronger as the binding energy is calculated as 0.58 eV. Note that, as binding between Mn and NO is still much stronger than tip− NO interaction, this tip-induced manipulation is too weak to break the Mn−NO bond. Interestingly, along with the structural changes, the spin state, which remains as S = 0 for MnP−NOlinear−Au4, becomes S = 1 for MnP−NObent−Au4. Moreover, even if the tip is removed, the MnP−NO geometry remains in its bent metastable state where ∠Mn−N−O ≈ 143° and spin state S = 1. Thus, by mechanically bending the linear Mn−N−O bond, one can switch the spin state of MnP−NO from off to on state. Atomistic understanding about this STM tip induced conformational as well as magnetic-state manipulation is explained in detail in the Supporting Information. Electron Transport Properties. In the following section, we explore how the electron transmission gets modified along with the conformational change in MnP−NO when it remains sandwiched between two gold electrodes (i.e., E−M−E system). However, before that, we make sure that localizedorbital basis set incorporated code, SIESTA (used for further calculations),51 qualitatively reproduces the VASP-calculated electronic and magnetic properties of MnP−NO (see Figure S9 in the Supporting Information). We describe the detail of E− M−E structure and computational processes in the Supporting Information. As shown in Figure 6, parts a and b, the transmission function near the EF gets modified substantially as the STM tip induces conformational change in MnP−NO. Notably, the transmission function, which is spin-independent for MnP− NOlinear-based molecular bridge, switches on strong spinpolarized nature in its bent conformation. When these transmission functions along with their corresponding DOS and molecular pDOS are analyzed, as shown in Figure 6a−d, transmission peaks exhibit prominent correlation with the peaks of molecular pDOS spectra. Fundamentally, transmission peaks appear only when narrow molecular states resonate with the states of electrodes. Now, for MnP−NOlinear, as all the molecular states are spin-independent (i.e., either completely filled or completely empty), transmission through these states obviously remains spin-unpolarized. Further, near to the EF, the transmission spectra shows prominent resonating peaks, i.e., peaks 1 and 2. As depicted in Figure 7, parts a and b, the charge densities of the eigenstates of the corresponding transmission peaks are completely delocalized over the whole molecular bridge and form a suitable resonating channel for carrier transmission between the two electrodes. Along with π orbitals of the conjugated porphyrin ring, 3dxy and 3dz2 orbitals of Mn

predefined path has been implemented to induce precise changes in the molecular geometry.48 We model the gold tip by taking a pyramidal cluster, containing four Au atoms in it (see Figure S5, parts a and b, in the Supporting Information). This bare Au4 cluster geometry has been optimized without any constraint and kept fixed for further calculations. This cluster model has successfully been used in several earlier studies to mimic the effect of an STM tip.49,50 We also optimized the Au4 coordinates along with MnP−NO molecules but do not find any major difference in the results (see Figure S6 in the Supporting Information). Further test calculations considering a larger Au cluster with 10 Au atoms (see Figure S5, parts c and d, in the Supporting Information) eventually produces the same results (see Figure S7 in the Supporting Information). We propose the following scheme to realize STM tip assisted spin switching in MnP−NO: initially, the metalloporphyrin molecule can be deposited on the Au(111) surface at low temperature and ultrahigh-vacuum condition as suggested by several previous reports.19,20 Then gaseous NO molecules can be exposed to the adsorbed MnP. In the ground state, NO binds to the Mn center in linear mode as discussed earlier. Now, using the STM imaging, the Mn−NO bond can be exactly located. At cryogenic temperature, the STM tip should be placed so close to the O of NO that the tip-apex Au atom (Autip) can exert a chemical binding force on oxygen.48 Further, lateral movement of the tip in constant height mode efficiently modifies the binding of Mn−NO from ground-state linear to metastable bent conformation and vice versa. Lastly, the tip can be removed from the vicinity of Mn−NO after achieving the desired molecular geometry as both the conformers are quite stable at low temperature. To find the characteristic threshold tip height (i.e., the tip− ON distance where the interaction is strong enough for mechanical manipulation),48 we relaxed the isolated Au4−ON cluster and consider the required height the same as the optimized Au−O bond length, i.e., 2.18 Å. Further, to simulate the STM tip assisted conformational change, we first optimize the geometry of MnP−NOlinear by keeping the STM tip (MnP−NOlinear−Au4) fixed at the threshold height. NO remains almost linearly bonded to Mn, i.e., ∠Mn−N−O ≈ 168°. Binding energy between MnP−NOlinear and Au4 also appears to be weak, i.e., 0.08 eV only. Further, we shift the tip laterally by 1 Å through the middle of two Mn−Npyrrole bonds. With this precise manipulation, the Mn−NO bond modifies its geometry to the bent conformation. As shown in Figure 5b, in MnP−NO bent −Au 4 we find ∠Mn−N−O ≈ 132° and E


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analysis about this spin polarization is given in the Supporting Information. As the real molecular devices operate under finite bias, we calculate the current (I) through the molecular junction by varying source−drain bias voltage (VSD). As shown in Figure 8a, total current through this molecular junction remains almost

Figure 6. Top panel: zero-bias transmission functions for (a) Au− [MnP−NOlinear(S2)]−Au (spin-unresolved) and (b) Au−[MnP− NObent(S2)]−Au (spin-resolved). Solid black and red lines represent the majority- and minority-spin transmission function. Major transmission peaks are numbered as 1−4. Bottom panel: DOS for (c) Au− [MnP−NOlinear(S2)]−Au (spin-unresolved) and (d) Au−[MnP− NObent(S2)]−Au (spin-resolved). Solid green portions depict the pDOS of molecular bridges of E−M−E systems.

Figure 8. (a) I−V characteristics for Au−[MnP−NOlinear(S2)]−Au and Au−[MnP−NObent(S2)]−Au. (b) I−V characteristics for Au− [MnP−NObent(S2)]−Au after shifting the electrode EF to −0.737 eV. (Imaj)/(Imin) is shown in the inset of panel b.

unchanged in the two different Mn−NO binding conformations. Further, in bent conformation, as majority- and minorityspin current remain almost equal to each other, the net transmitted current shows spin-unpolarized behavior. However, for this conformer, when the EF of E−M−E is shifted to −0.737 eV, spin-polarized current appears. As depicted in Figure 8b, now majority-spin current (Imaj) clearly dominates over the minority-spin current (Imin) at low voltage (i.e., 0 ⩽ V ⩽ 0.5). Further, (Imaj)/(Imin) reaches up to ≈12 at infinitesimal voltage and reduces with increase in applied bias voltage (see the inset of Figure 8b). Fundamentally, as peak 3 appears at −0.737 eV, the majority-spin channel predominantly gets activated for transport upon alignment of the EF at that energy state. And consequently, the molecular junction becomes a highly efficient spin-polarized current generator at the low-voltage regime. The same kind of spin-polarized current can be generated by precisely aligning the system’s EF to other spin-polarized transmission peaks also. Thermoelectric Properties. Very recently, apart from electronic transport, these molecular junctions also exhibited their promising applications in the field of thermoelectrics.52,53 Concurrent study of conductance and thermoelectric properties in similar kinds of E−M−E systems strongly encouraged us to explore the spin-dependent thermoelectric effects of the present nanojunction.13 We use the Landauer formula to calculate the thermally induced current which appears due to the temperature difference in two electrodes. The temperature difference between two electrodes is denoted as TSD = TS − TD, where TS (TD) is the temperature of the source (drain) electrode and TS > TD. The spin-resolved current for σ-spin (i.e., majority/ minority) is formulated as9

Figure 7. (a−d) Wave function plot of the states which corresponds to the transmission peaks 1−4 of Figure 6. The isosurface value is considered as 0.01 e/Å3.

majorly contribute to these transport-active molecular channels. Other occupied molecular orbitals stay far from the EF and consequently remain quite inactive toward molecular transport. Importantly, as the eigenstates at the EF remain majorly localized over the electrodes, the transmission spectra show a sharp deep at that energy (see Figure S11 in the Supporting Information). On the other hand, transmission spectra for the MnP− NObent bridged junction exhibit a quite distinct nature between major and minor spin channels just below the EF. In Figure 6b, the minority (majority) spin channel shows a sharp transmission peak, i.e., peak 4 (peak 3) at −0.16 eV (−0.74 eV). Alike linear geometry, completely delocalized charge densities of the eigenstates result in the major peaks in transmission spectra. As shown in Figure 7c (Figure 7d), charge density for the corresponding eigenstate of peak 3 (peak 4) has dominant contributions from the conjugated π orbitals of porphyrin and majority-spin orbitals of 3dxy and 3dz2 (minority-spin orbitals of 3dxy, 3dyz, and 3dz2) of Mn. Thus, in bent Mn−NO conformation, spin-split 3d orbitals of Mn result in spin polarization of electron transmission spectra. Quantitative

Ithermo, σ =

⎛e⎞ ⎜ ⎟ ⎝h⎠

∫ τσ[fS (TS , E) − fD (TD , E)] dE

(3)

where f S (f D) is the Fermi distribution function of the source (drain) electrode. Further, charge and spin current of the device are evaluated as Icharge = Imaj + Imin and Ispin = Imaj − Imin, respectively. In the linear response regime, we formulate spinresolved Seebeck coefficient as F


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Sσ = −

1 L1, σ (T , μ) eT L0, σ (T , μ)

switches on as a measurable amount of current starts flowing through the junction. At TS ≈ 400 K and TSD = 80 K, the Ion/ Ioff ratio reaches as high as 105 for the charge current (Figure 9d). Thus, the device can be demonstrated as a temperaturecontrolled high-performance switch. Further details of the switching phenomenon are discussed in the Supporting Information. However, the molecular junction of MnP−NObent exhibits quite distinct features in thermally induced current transport. Most importantly, unlike the previous conformation, spin degeneracy gets lifted in this metalloporphyrin junction, resulting in the appearance of spin-polarized current (Figure 10a). Further, Ithermo,bent for majority-spin as well as minority-

(4)

1

where Ln , σ(T , μ) = h ∫ τσ(E)(E − μ)n [δ Ef (E , μ , T )] dE , T is the temperature of the whole system, and μ is the chemical potential of the electrodes.9 To evaluate the charge Seebeck coefficient SC, we consider total transmission τtot = ∑στσ, whereas the spin-polarized Seebeck SS is calculated by using the equation SS = Smaj − Smin, where Smaj (Smin) is the majority-spin (minority-spin) Seebeck coefficient.9 Temperature-Induced Current. First, we calculate the current (Ithermo,linear/bent) for both metalloporphyrin-based molecular bridges as a function of TS (100 K ⩽ TS ⩽ 400 K) at different TSD. As shown in Figure 9a, total charge current

Figure 10. (a) Ithermo,bent vs TS with various TSD; J(E) as a function of energy with various (b) TSD; (c) spin-resolved log(|ISD|) vs TS plot with TSD = 80 K; (d) spin polarization of Ithermo,bent vs TS for various TSD.

Figure 9. (a) Ithermo,linear vs TS with various TSD; J(E) as a function of energy with various (b) TSD and (c) TS; (d) log(|ISD|) vs TS plot with TSD = 80 K.

spin currents show up a negative sign. The spin-dependent transmission spectra (Figure 6b) and current spectra (see Figure 10b for majority-spin current spectrum) demonstrate that, for both the spin channels, electrons majorly transport thermal current. Thus, electrons flow from hot source to cold drain electrodes and result in Ie. Further, at TS = 400 K and TSD = 80 K, the majority-spin (minority-spin) channel reaches a maximum Ion/Ioff ratio of 106 (106), which is an order higher than the previously explored device (Figure 10c). Focusing on spin polarization of Ithermo,bent, we find the appearance of minority-spin-polarized resultant current in this molecular junction. As shown in Figure 10d, the resulting spin-polarized current also increases with the increment in TS (TSD). In the Supporting Information, we explain the appearance of spin polarization of current in detail. Thermopower. Here, we explore the relationship between SC (SC and SS) and chemical potential (μ) of the electrodes for the MnP−NOlinear (MnP−NObent) bridged molecular junctions. Note that, during the thermopower measurements, VSD = 0, and consequently, the chemical potential of the electrodes (i.e., μL, μR) remains aligned to the equilibrium EF of the system. Further, restricting this present investigation within the linear response regime, we keep TS ≈ TD ≈ 300 K and vary μ in the range of −1 ⩽ μ ⩽ 1. Equation 4 clearly depicts that alike current, thermopower generation is also strongly governed by the features of spin-resolved transmission spectra near the EF. Thus, this nanodevice prominently alters SC and SS during the

(Ithermo,linear) through the MnP−NOlinear bridge increases with TS for a constant TSD value. Current also increases in the presence of higher temperature bias, i.e., TSD, as already has been found in experiments.13 Fundamentally, as pointed out in the eq 3, the magnitude and sign of Ithermo,linear at the low-bias regime is determined by two terms: (1) the relative difference in the Fermi distributions of two electrodes, i.e., [f S(TS, E) − f D(TD, E)], and (2) the transmission function (i.e., τσ) of the molecular junction near the EF. Due to the temperature difference, electrons (holes) of the source (drain) electrode, with energy higher (lower) than EF, get transported to the drain (source). And consequently, the device exhibits negatively (positively) signed electron (hole) current Ie (Ih) which flows from the drain to the source electrode. Now, the transmission spectrum in Figure 6a and current spectrum (i.e., J(E) = τσ[f S(TS, E) − f D(TD, E)]) in Figure 9, parts b and c, clearly show that transmitting states below EF are the major contributors of hole-dominating current in this junction. Thus, the resultant positively signed current flows from source to drain for this bias region. Further, as shown in Figure 9a−c, increase in either TS or TSD eventually results in activation of more transport channels below the EF, and that increases the magnitude of Ithermo,linear. Interestingly, Figure 9a shows a negligible amount of current at TS = 100 K (Ithermo,linear < −0.004 μA), where the device remains in the off state. Now if we increase the TS maintaining a certain TSD, the device G


Article


junction in its bent conformation, one can find pure spin thermopower, which is highly demanding in the field of spin caloritronics. As the present theoretical calculations are performed within the constraint of a well-developed experimental setup, we strongly believe that the exciting results of this study can be verified experimentally.

conformational change of Mn−NO geometry. Parts a and b of Figure 11 further demonstrate that (1) due to the presence of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11227. Discussion about parametrization of U, optimized structures and spin densities of all TMP molecules, optimized geometries of NO-coordinated VP, CrP, FeP, and CoP, reason for change in bond length of MnP−NO during conformational modification, pDOS of Mn in the MnP molecule, detail of interaction between MnP−NO and the Au(111) surface and between MnP−NO and the Au STM tip, optimized geometry of Au STM tips, geometries of MnP−NO with the Au tip, pDOS of Mn in bent form of MnP−NO using SIESTA and VASP, calculation details, optimized geometries and wave function square plot of E−M−E, spin polarization of transmission, switching and spin polarization of thermal current (PDF)

Figure 11. (a) SC(linear) and (b) SC(bent) and SS(bent) as a function of μ.

electron−hole symmetry, thermopower becomes almost zero at certain energy values, (2) when finite conductance and electron−hole asymmetry appear simultaneously, finite thermopower arises; (3) thermopower changes its sign when μ crosses peaks of transmission spectra as carrier-type changes there. Note that S becomes negative to positive when carrier changes from electrons to holes. Particularly, at μ = 0, SC for the MnP− NOlinear bridged junction (SC(linear)) appears as −35 μV/K, which indicates an electron-dominated thermopower generation (see Figure 11a). Note that, as transmission through the MnP−NOlinear junction remains spin-degenerate (i.e., τmaj = τmin), the SS(linear) becomes zero here. Further, in MnP−NObent conformation, the sign as well as magnitude of thermopower gets modified quite distinctly. Importantly, alike thermally induced current, spin-polarized Seebeck (SS) gets switched on in this device as the transmission function becomes spin-nondegenerate (i.e., τmaj ≠ τmin). At μ = 0, SS and SC(bent) appear as −13.2 and 26.6 μV/K. Thus, the device indicates the generation of hole-mediated charge thermopower at zero bias where both majority- and minorityspin channels have holes as the dominant carrier. Most importantly, Figure 11b demonstrates that at −0.95 eV ⩽ μ ⩽ −0.77 eV and −0.68 eV ⩽ μ ⩽ −0.44 eV, SS becomes much larger than SC. Fundamentally, in these regions, completely spin-polarized majority-spin holes and minority-spin electrons participate in thermopower generation, which results in a smaller magnitude of SC while a much larger value of SS. Specifically, at μ ≈ −0.9 eV and μ ≈ −0.5 eV, where SC = 0 but SS = −327 and −32.6 μV/K, respectively (see the green circles in Figure 11b), we can generate pure spin thermopower without any charge component.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.G. and S.K.P. acknowledge TUE-CMS, JNCASR for the computational facilities and DST for funding.

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REFERENCES

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CONCLUSIONS To conclude, in the present study, we have demonstrated the conformation-induced spin-state switching in an NO-adsorbed MnP molecule, adsorbed on a Au(111) surface. The low-spin (S = 0) ground state of linearly bonded NO containing MnP switches on its spin state to S = 1 when Mn−NO bonding moves to the metastable bent conformation. The mechanical manipulation of a closely spaced gold STM tip can precisely modify this Mn−NO bonding conformation in a reversible way. Electronic transport as well as thermoelectic properties of this molecular junction also gets significantly modified upon the conformational change. Most importantly, the spin-polarized nature of voltage- or temperature-induced current can reversibly be switched off/on by the structural manipulation. By precisely controlling the chemical potential of this molecular H


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