Two‐Terminal Molecular Memory through Reversible Switching of Quantum Interference Features in Tunneling Junctions

Abstract Large‐area molecular tunneling junctions comprising self‐assembled monolayers of redox‐active molecules are described that exhibit two‐terminal bias switching. The as‐prepared monolayers undergo partial charge transfer to the underlying metal substrate (Au, Pt, or Ag), which converts their cores from a quinoid to a hydroquinoid form. The resulting rearomatization converts the bond topology from a cross‐conjugated to a linearly conjugated π system. The cross‐conjugated form correlates to the appearance of an interference feature in the transmission spectrum that vanishes for the linearly conjugated form. Owing to the presence of electron‐withdrawing nitrile groups, the reduction potential and the interference feature lie close to the work function and Fermi level of the metallic substrate. We exploited the relationship between conjugation patterns and quantum interference to create nonvolatile memory in proto‐devices using eutectic Ga–In as the top contact.

Quantum interference (QI) is ac ollection of phenomena related to fermions,w hose wave functions can interfere with themselves;i nm olecular tunneling junctions,d estructive QI can lower the transmission probability between the electrodes,s ignificantly lowering conductance without altering the tunneling distance. [1] Thus,c ompounds that produce destructive QI could act as molecular switches,m emory devices,o r transistors. [2,3] Destructive QI effects have been studied both theoretically and in multiple experimental platforms.I npconjugated molecules,t hey are generally ascribed to crossconjugation, [4][5][6][7] meta-substitution, [8,9] or particular spatial arrangements. [10,11] Of particular interest are systems capable of toggling QI effects through external inputs; [12][13][14] however, control over QI effects is currently limited to transient, singlemolecule junctions and/or comparisons of different compounds in different environments, [15][16][17] for example,t he ex operando (electro)chemical interconversion between ac ross-conjugated quinone and linearly conjugated hydroquinone. [18,19] Herein we show that self-assembled monolayers (SAMs) of ac ross-conjugated compound incorporating at etracyanoquinodimethane (TCNQ) unit, TCNAQ (Figure 1), on different metal substrates can be switched between, and addressed in, two conductance states (ON and OFF) in at wo-terminal proto-device using eutectic Ga-In (EGaIn) top contacts.W eascribe the different conductance states to the modulation of the bond topology of the molecule; TCNAQ-just as TCNQ-can readily accept an electron (see Figure S4 in the Supporting Information) and form as table (di)anion that converts cross-conjugated pathways to linearly conjugated pathways,t hus altering the transmission probability similarly to the interconversion of quinones and hydroquinones ( Figure 1). [20] Al ow-lying LUMO brings the reduction potential of TCNAQ close to the oxidation potential of Au,A g, and Pt electrodes,t hus eliminating the need for athird electrode or redox agents.
We prepared SAMs of TCNAQ on Au,A g, and Pt surfaces from the thioacetate precursor through in situ deprotection. [21] X-ray photoelectron spectroscopy (XPS) spectra were consistent with upright-standing molecules attached to the surface through as ingle thiolate bond ( Figure 2). Synchrotron measurements provide additional evidence that the nitrile groups were oriented predominantly parallel to the substrate (see the Supporting Information for details). TheN1s region of the XPS spectrum of the SAM features an additional peak at lower binding energy (398.5 eV) that is not present in spectra of TCNAQ powder, which we ascribe to the spontaneous (partial) reduction of TCNAQ by the metal substrate.Similar shifts are common in monolayers of TCNQ that are spontaneously reduced by the underlying metal. [22] There,TCNQ is directly adsorbed to the metal substrate,w hereas in SAMs of TCNAQ,t he redoxactive core is bound through ap henylacetylene arm that is coupled to the surface through ac ovalent SÀAu bond. Thus, charge transfer (redox) can still occur in ag eometry that is compatible with the formation of metal-molecule-metal junctions.I nt he XPS spectra, about 14 %o fTCNAQ molecules in the SAM are in ar educed state.
We investigated the electrical properties of TCNAQ in Au-on-mica/SAM//EGaIn junctions (where "/" and "//" denote covalent and Va nder Waals interaction, respectively). EGaIn is al iquid metal that can be used to form stable, conformal, nondamaging contacts with SAMs with adiameter of about 20 mm. [23][24][25] This methodology enables the formation of junctions in multiple areas of as ubstrate rapidly and reproducibly,t hus allowing the collection of statistically significant data and spectroscopic investigation of the SAM after J/V cycling. [26] As controls,w em easured junctions comprising hexadecanethiol (C16SH)a nd analogues of TCNAQ bearing an anthraquinone core (AQ)o ralinearly conjugated, non-redoxactive anthracene core (AC ;see Figure S1). Figure 3s hows forward and reverse J/V traces for junctions comprising TCNAQ (A), AC (B), C16SH (C), and AQ (D). While the J/V traces of the latter three junctions overlap perfectly, TCNAQ exhibits ah ysteresis loop at negative bias;t hat is, after being biased at positive voltages,t he conductance at negative bias decreases (OFF) and then recovers its initial conductance (ON) after reaching À1.00 V. Am aximum ratio of J between forward and reverse scans of 2.6 occurred at À0.65 V. Thehysteresis and magnitude of switching was reproducible across junctions comprising TCNAQ on Au-on-mica and template-stripped [27] (TS) Au TS and Ag TS (Figure 4). Theeffect was present but diminished on Pt TS (see Figures S27 and S28). No hysteresis or switching was present on any substrate for AC, AQ,orC16SH.
Given sufficient trace/retrace stability,hysteresis is aform of two-terminal bias switching, [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] but to translate it into am emory effect, the state of as tatic,t wo-terminal junction must be switched reversibly between at least two conductance states in operando.T oc haracterize the memory effects of metal/SAM//EGaIn junctions,weperformed write operations (W) by applying a À1.50 Vbias,which puts the junction in the high-conductance ON state,a nd erase operations (E) by applying 1.00 V, which puts it in the low-conductance OFF state.W er ead the state at À0.50 V, measuring current densities J of 0.10-0.01 Acm À2 .F igure 5A compares the resulting ON/OFF ratios for junctions comprising SAMs of TCNAQ, AC, AQ,a nd C16SH on Au over four switching cycles.A se xpected, the ON/OFF ratio for the controls (AC, AQ,a nd C16SH)w as 1, indicating no effect. However, TCNAQ exhibited ratios as high as 6 AE 2. Thememory effect was identical for Au-on-mica and Au TS ,but the peak switching ratio decreased and shifted to less negative bias on Ag TS ( Figure 5A). On Pt TS ,the peak decreased and shifted further, and the hysteresis became noticeable only over al arger bias window (see the Supporting Information for details). This trend is consistent with the proposed mechanism because the magnitude of the suppression of conductance scales with the proximity of the destructive QI feature to the Fermi level. [44] As shown in Figure 5, the ON/OFF ratio of TCNAQ slowly decreased, approaching 2a fter about 10 cycles; however, even after 30 cycles,a pplying a À2.00 Vb ias restored the initial ON/OFF ratio (see Figure S34). The conductance of TCNAQ decreased with each W-E cycle (see   Table S3 in the Supporting Information), which is typical for repeated switching cycles in molecular tunneling junctions. [15] Forj unctions comprising TCNAQ,s uch damping could be the result of irreversible reactions,s tructural modification of the SAMs,o rachange in the EGaIn contact. However, because the conductance remained constant during J/V sweeps (Figure 3, as opposed to R-E-W cycles,i .e., Figure 5), the underlying phenomenon is specific to large,r apid changes in bias.
In its pristine state, ad estructive QI feature is clearly present in the transmission spectrum near E f (see Figure S44 A). [45] In single-molecule STM break-junctions comprising TCNAQ,t his QI feature manifests as al ow conductance,c omparable to that of AQ,w hich is known to exhibit strong destructive QI effects in both single-molecule and SAM-based junctions (see Figure S41). [6,46] However, ensemble junctions comprising SAMs of TCNAQ exhibit ar elatively high conductance,c omparable to that of the linearly conjugated analogue AC (see Figure S10), which does not exhibit any QI features.
We ascribe the difference between TCNAQ in SAMs and in single-molecule junctions to the presence of reduced TCNAQ in the SAM ( Figure 2C). [47] Tu nneling charge transport through SAMs is sensitive to the entire supramolecular structure of the monolayer,w hich comprises molecules in different conformations and, in the case of TCNAQ,r edox states. [11,48] Figure 1B shows the bond topology of TCNAQ in the pristine and reduced states.T he addition of one or two electrons converts the cross-conjugated quinoid core into al inearly conjugated, fully aromatic hydroquinoid. (The   driving force of rearomatization is the reason that TCNQ is an exceptional electron acceptor.) Tr eating each molecule in aS AM as ar esistor in parallel, it follows from the Kirchoff rules that asmall fraction of reduced TCNAQ molecules can dominate charge transport through the SAM owing to the exponential difference in the conductance of TCNAQ in the cross-and linearly conjugated (quinoid/hydroquinoid) forms. [49] Specifically,i ft wo pathways in aS AM differ in conductance by two orders of magnitude (similar to AQ and AC at 0.50 V), the presence of only 1-2 %o ft he more conductive pathways is sufficient to dominate the conductivity of the SAM. [50] Thus,t he hysteresis and switching phenomenon are most likely caused by as hift in the equilibrium between the low-and high-conductance states of TCNAQ;applying abias to the junction affects the fraction of molecules in the junction that exhibit destructive QI.
If the proposed mechanism is correct, the ON state is metastable and should slowly relax to lower conductance, since the thermodynamic minimum is the neutral, lowconductance state.I ndeed, the ON state current decreases in time with multiple read cycles,whereas the OFF state only shows small, stochastic fluctuations,w hich are discussed further in the Supporting Information. Thei nitial ON/OFF ratio is restored after anew Wcycle;that is,the application of apulse at negative bias restores the SAM to the initial state,in which agreater fraction of TCNAQ is in the reduced state.We ascribe this observation to the slow kinetics of the relaxation (reoxidation) process.W henacharge is placed in aS AM by the reduction of am olecule,t he local environment in the SAM reorganizes to minimize the free energy of the system, which is am uch slower process than the initial electron transfer. Within 5min, without any applied bias,the ON/OFF ratio decreases to 70 %ofits initial value;after 20 min it has decreased to 60 %.
Metal/TCNAQ//EGaIn junctions are af orm of nonvolatile memory;t heir state is retained when the power (bias) is removed. It is difficult to contextualize TCNAQ further.
There are examples of memory effects in molecular tunneling junctions,each demonstrating asalient feature:Some exhibit high switching ratios as single molecules,b ut not in (proto-)device platforms; [37,42] some require prohibitively complex fabrication; [35] some only switch at low temperature; [28] some are resistant to fatigue when switched with light, but not with bias. [51] In simple,two-terminal proto-devices, TCNAQ exhibits reasonably high ON/OFF ratios that are stable for tens of minutes and that can be refreshed or rewritten over at least dozens of cycles.T he switching mechanism is phenomenologically unique,e xploiting the coincidental alignment of ad estructive QI feature and facile reduction with the Fermi level and work function of Au to enable the shift of adynamic equilibrium of molecules in high-conductance states lacking QI features and low-conductance states with strong QI features near the Fermi level. Thes witching effect is molecular in nature,a nd further investigation and optimization could feasibly exploit this type of QI-based switching to achieve switching ratios of orders of magnitude.