Principles of design for substrate-supported molecular switches based

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Principles of design for substrate-supported molecular switches based on physisorbed and chemisorbed states Sha Yang, Shuang Li, Sergey N. Filimonov, Miguel Fuentes-Cabrera, and Wei Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07568 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 15, 2018

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ACS Applied Materials & Interfaces

Principles of design for substrate-supported molecular switches based on physisorbed and chemisorbed states Sha Yang,† Shuang Li,† Sergey N. Filimonov,‡ Miguel Fuentes-Cabrera,¶ and Wei Liu*,† †Nano and Heterogeneous Materials Center, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, China ‡Department of Physics, Tomsk State University, 634050 Tomsk, Russia

¶Center for Nanophase Materials Sciences, and Computational Sciences and Engineering

Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States KEYWORDS: Molecular switches, organic-metal interface, van der Waals forces, precursor state, conformational switching Abstract The physisorbed (precursor) and chemisorbed states of a molecule on metal surfaces can be utilized

to

build

a

logic

switch

at

the

single-molecule

level,

enabling

further

microminiaturization of electronic devices beyond the silicon limits. However, a serious drawback of this design is easy lateral diffusion of the molecule in the physisorbed state, which may destroy the normal switch operation. Here, we demonstrate that anchoring engineering can

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be an effective way to enhance the stability of molecular switches without degrading switching functionality. As exemplified by trans-ADT on Cu(111), we show that the lateral diffusion of such molecular switch can be obstructed by the anchoring of the ending thiophene groups, along with a rotation of the adsorbate during the switching process. More general, our results also suggest that when searching for molecular switches with reversible physisorbed and chemisorbed states with excellent bistability and lateral stability, the focus should be on finding molecules with a moderate HOMO-LUMO energy gap and anchoring atoms with positive charge, that can then be deposited on substrates with which they interact moderately. This allows further improvement of the lateral and vertical stability of such molecular switch by substituting the thiophene groups with selenophene, thus establishing trans-ADS on Cu(111) as a promising switch.


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Introduction A single molecule that performs as a logic switch provides a much higher degree of device integration than conventional silicon-based electronic components.1-3 Such a molecular switch must possess at least two distinguishable and stable states that can be repeatedly switched back and forth. Over the last few decades, various molecular switch architectures based on mechanisms like atomic spin,4,5 photosensitiveness,6-8 chirality,9,10 charge state,11-13 and tautomerization14-16 have been developed. However, molecular backbones usually endure drastic conformational changes during switching, which may significantly alter the molecular frame and even destroy the reliability of the system.17 To solve this problem, recently some of us proposed a novel push-button molecular switch from benzene derivatives deposited on metal surfaces. In this case, upon switching the conformational molecular change is relatively small because the switching process only involves an adsorption-induced deformation at the physisorbed (precursor) and chemisorbed states.18-20 However, one of the drawbacks of this design is that the diffusion barrier at the precursor state is lower than the switching barrier; this may lead to the lateral diffusion of the molecule during the triggering process. The theoretically proposed concept of the push-button molecular switch based on the reversible transition between the physisorbed and chemisorbed states of a molecule was experimentally realized by Kern′s group,21 who deposited anthra-dithiophene (ADT) on Cu(111) and observed two states of adsorption with distinct conductance. Two isomers of ADT, namely cis- and trans-ADT, are shown in Chart 1, where the thiophene rings are fused at two ends of the acene core.22 Scanning tunneling microscope (STM) experiments showed that both isomers can be switched back and forth from the physisorbed (high-conductance) to chemisorbed (lowconductance) state by applying a positive or negative bias at the center of the molecule.21 They


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also showed that the physisorbed “on” state spontaneously converts to the chemisorbed “off” state;21 that the switching of cis-ADT, but not trans-ADT,23,24 can be remotely triggered by the injection of holes or electrons into the Cu(111) surface; and, finally, that as compared to cisADT, trans-ADT was not only more stable but also underwent a rotational movement upon switching. However, fundamental understanding of the mechanisms underlying the rotational movement, the isomer selective switching, and the role of sulfur atoms in the process of transformations, remained unclear. The objective of this study is to shed light onto these mechanisms, and use this knowledge to design improved molecular switches with reversible physisorbed and chemisorbed states. S

S

Se Se

cis-ADT (anthra[2,3-b:7,6-b´]dithiophene)

trans-BDS (benzo[1,2-b:4,5-b´]diselenophene)

S

Se Se

S trans-ADT (anthra[2,3-b:6,7-b´]dithiophene)

trans-NDS (naphtho[2,3-b:6,7-b´]diselenophene) Se

O

Se

O trans-ADF (anthra[2,3-b:6,7-b´]difuran)

trans-ADS (anthra[2,3-b:6,7-b´]diselenophene)

Chart 1. Structures of the cis-ADT, trans-isomers of the ADT, ADF, BDS, NDS, and ADS molecules. We start by carrying out detailed dispersion-inclusive density-functional theory (DFT) calculations to study the structure, energetics, and the switching mechanisms of ADT. We found that i) the conductance switching of trans-ADT on Cu(111) is originated from the central carbon ring, whereas the ending thiophene groups determine the adsorption orientation; ii) the binding between S and Cu atoms in trans-ADT produces an anchoring effect that causes a large barrier to


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lateral diffusion that surpasses even the barrier to switching; and iii) the switching ratio not only depends largely on the energy gap of the molecules, but also has a magnitude that can be determined by the difference in the adsorption height of the two states. We then set out to improve the switching performance of trans-ADT by investigating molecular analogues, such as benzo-diselenophene (BDS), naphtha-diselenophene (NDS), tetra-diselenophene (TDS), anthradiselenophene (ADS), and anthra-difuran (ADF) on Cu and other metal surfaces including Ag/Au/Pd/Pt/Rh/Ir(111). In this manner, we find that substituting the thiophene groups in transADT by selenophene increases the anchoring effects, as well as the lateral and vertical stability; this leads us to propose an improved molecular switch based on the trans-ADS/Cu(111) system. Results and Discussion The adsorption of cis-ADT on the Cu(111) surface was studied using the dispersion-inclusive DFT+vdWsurf method,25 coupled to the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Note that, the PBE+vdWsurf method has been extensively applied to both chemisorbed and physisorbed systems and shown to predict accurately the structure and stability of organic/metal interfaces.26,27 As shown in Figure 1a, in the energetically most favorable adsorption configuration of cis-ADT, the long axis of the molecule is along the close-packed direction of Cu(111), similar to pentacene on Cu(111).28 The dumb-bell shape seen in the simulated STM image (Figure 1b) is consistent with the “off” state of cis-ADT on Cu(111) observed experimentally.21 The binding energies were obtained as a function of the adsorption heights of the two central C atoms (termed as potential energy curve). Figure 1c shows a stable “off” state at adsorption height of 2.2 Å and an incipient precursor state at 2.7 Å, similar to the adsorption of benzene on Pt(111).20 The barrier of the off-to-on switching for cis-ADT on Cu(111) is estimated to be 0.03 eV from the potential energy curve, which would trap the


5


molecule at the “off” state up to a temperature of 10-14 K (the computational details refer to the Methods section). Moreover, the incipient precursor state in the potential energy surface indicates that the “on” state is unstable, in agreement with the decay experiments, in which the “on” state spontaneously converts to the “off” state at 6 K.21 Given the poor bistability of cisADT on Cu(111), and thus its limited applicability as a molecular switch, from now on we focus on the trans-ADT/Cu(111) system.

[112]

a [110] 2.26 Å

2.54 Å

2.19 Å

cis-ADT (off)

b

2.54 Å

2.81 Å

trans-ADT (off)

2.55 Å

trans-ADT (on)

High 10.9° Low _2.60

d

_2.60

_2.65

_2.65

_2.70

_2.70

_2.75

_2.75

Eads (eV)

c

Eads (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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_2.80 _2.85 _2.90 _2.95 _3.00

cis-ADT/Cu(111)

2.0 2.2 2.4 2.6 2.8 3.0 h (Å)

off on

_2.80 _2.85

10.9° 10.9

_2.90

b

_2.95 _3.00

a 2.0 2.2 2.4 2.6 2.8 3.0 h (Å)

Figure 1. Adsorption geometries and energies. (a) Adsorption geometries for cis- and trans-ADT on Cu(111). The heights of the central C and side S atoms were calculated relative to the position of the unrelaxed topmost metal layer. (b) Simulated STM images for trans-ADT on Cu(111) at the “off” and “on” states, obtained using the Tersoff-Hamann approximation,29,30 are in good agreement with the experimental observations.21 (c) Potential energy curve for cis-ADT on Cu(111). The binding energies as a function of adsorption height of cis-ADT on Cu(111) were calculated by constraining the two central C atoms along the z axis (perpendicular to the surface), while the rest of the atoms were fully relaxed. The insert represents simulated STM image at adsorption height of 2.7 Å, similar to the experimental STM image at the “on” state.21 (d) Potential energy curves for trans-ADT on Cu(111) at the “off” (blue) and “on” (red) states.


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The blue (red) background represents the energetic preference of the “off” state (“on” state) at the particular adsorption height range. In contrast to cis-ADT, trans-ADT exhibits two stable states on Cu(111). As shown in Figure 1a, the two states correspond to different adsorption heights and orientations, and both states are distinguishable from the simulated STM images. Figure 1b shows a dumb-bell shape at the “off” state and a planar shape at the “on” state. Trans-ADT on Cu(111) exhibits excellent bistability due to the tradeoff between the vdW interactions and the deformation energy (c.f. Table 1). Both the vdW forces and deformation energy are larger at the chemisorbed “off” state than at the physisorbed “on” state, leading to an almost equal stability of the two states. However, the adsorption energy at the “off” state is slightly larger than at the “on” state, in agreement with the experimental observations that, upon deposition on Cu(111) at 200 K, transADT molecules adsorbed at the “off” state.21 Remarkably, the appearance of the “off” state is found to be related to the screening of dispersive interactions in the bulk metal; this becomes evident when the “off” state cannot be reproduced with the PBE+vdW method31 (Figure S2), which neglects the dielectric screening in bulk metals.32-37 We also notice that the switching function of trans-ADT on Cu (111) are highly dependent on the choice of functional. For example, the many-body dispersion interactions inclusive method, termed as PBE+MBD,38 does not reproduce the physisorbed state despite its improved description of the vdW interactions (see Fig. S2). However, as demonstrated in our recent study the accurate potential energy curve for adsorption systems can only be obtained when the MBD method is coupled to the hybrid functionals, such as Heyd-Scuseria-Ernzerhof (HSE).32,39,40 Therefore, further confirmation of these findings with functionals that provide an improved description of Pauli repulsion, such as HSE06, would be needed.


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Table 1. Lateral diffusion barriers Elat (eV), adsorption energies Eads (eV), adsorption heights of the two central C atoms h1 (Å) and side O, S, and Se atoms h2 (Å), contributions of vdW sub interactions EvdW (eV), deformation energies of the adsorbates Emol def (eV) and substrate Edef (eV) for trans-ADF, -ADT, and -ADS on Cu(111), atomic charge N (|e|) of O, S, and Se atoms of free molecule, respectively. The lateral diffusion barriers were evaluated from the lateral potential energy curve, consistent with the results from climbing image nudged elastic band method41 and the relative diffusion paths refer to Figure S3. The Bader charge analysis was used to calculate the atomic charge of the free molecules. Molecule

State

Elat

Eads

h1

h2

EvdW

Emol def

sub Edef

off

0.20 −2.43 2.58

2.96

2.82

0.10

0.02

on

0.11 −2.36 2.79

2.95

2.69

0.06

0.02

off

0.44 −2.86 2.19

2.53

3.47

0.50

0.12

on

0.20 −2.77 2.81

2.55

3.10

0.08

0.05

off

0.55 −3.09 2.20

2.58

3.55

0.48

0.10

on

0.23 −3.02 2.81

2.56

3.20

0.08

0.05

trans-ADF

trans-ADT

trans-ADS

N −1.04

+0.15

+0.31

In agreement with the experimental results,23 we find a 10.9° rotation of trans-ADT upon transition from the “off” to the “on” state (Figure 1b). This is different from previous pushbutton molecular switches where “off” and “on” states were located at the same position.20 The rotation of trans-ADT can be attributed to the matching between S atoms and the substrate: as illustrated in Figure 1a, for both the “off” and “on” states, the two S atoms rest just above Cu atoms, similar to prior theoretical studies for thiophene on Cu(111).42 Interestingly, the previous study carried out with the vdW-DF-cx method43,44 predicted a different adsorption configuration in the “off” state with one S atom located at the top site while the other is located at the bridge site.21 This configuration is however confirmed to be unstable in our PBE+vdWsurf calculations. Using the adsorption structure from the vdW-DF-cx method as the starting geometry, the bridgesite S atom is found to move to the top position upon full relaxations from PBE+vdWsurf. Due to


8

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the preferred location of the S atoms above the Cu atoms, trans-ADT can adsorb along orientations a and b of Cu(111) (Figure 1d; Fig S7), which ensure excellent match of the S atoms with the substrate (Table S1). In contrast, the S atoms in cis-ADT can only match well with the substrate along the direction [11 0] of Cu(111) (Figure 1a; Figure S7), which explains the absence of rotation in this system. In addition to the degree of matching between S and Cu atoms, energetic differences between “on” and “off” also contribute to the rotation. As shown in Figure 1d, an energy minimum appears at adsorption height of 2.20 Å with molecular orientation a (“off”), while a stable state exists at 2.80 Å along molecular orientation b (“on”). Thus, at adsorption heights above 2.62 Å, trans-ADT prefers to adsorb along the orientation given by the “on” state, whereas below 2.62 Å it acquires the orientation given by the “off” state. Because the two states have different orientation, a rotational movement ensues upon switching between one and the other. Because of the rotational movement of trans-ADT on Cu(111) during the switching, we propose two possible switching pathways. These are denoted in Figure 2 as one-step, path A, and two-step, path B. The switching barriers for the one-step and two-step transitions are found to be comparable, i.e., 0.17 vs. 0.15 eV, which were determined from the potential energy curves in Figure 1 and Figure S4 (the computational details refer to Figure S4). However, the slightly lower switching barrier involving path B makes it more energetically favorable. Along path B, holes injected into the molecule could disturb its potential energy landscape, and the molecule raises above 2.62 Å. Subsequently, the molecule rotates to acquire the orientation given by the more energetically favorable “on” state. This two-step switching is consistent with the experimental observations that a short-lived current appears in the switching process (corresponding to the “B1” state, see Figure 2), agreeing with their assumption that the molecular


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rotation results in a double-step current trace.21 Note that the switching barriers of the two pathways are significantly larger than that for the cis-isomer. This finding explains the experimental observations that trans-ADT could not be switched when the carriers were injected into the substrate (i.e., remote control), not into the molecule. Remarkably, the lateral diffusion barriers, 0.44 and 0.20 eV at the “off” and “on” states, respectively (see Figure S3), are significantly larger than the switching barrier, showing enhanced lateral stability compared with that for tetrachloropyrazine on Pt(111) (0.12 eV).20 We also find that the lateral diffusion barrier at the “off” state is larger than for pentacene on Cu(111) (0.25 eV), a consequence of the anchoring effect caused by the S atoms.

Path A lift & rotate

lift & rotate 0.17 eV off

A1

on

Path B lift

rotate

0.12 eV off

rotate

0.03 eV B1

B2

on

Figure 2. The off-to-on switching routes of trans-ADT on Cu(111). Path A is a one-step conversion, in which the lift and rotation of the molecule are involved in the whole switching procedure. State “A1” represents the transition state, and the switching barrier is 0.17 eV from the “off” state to the “A1” state (Figure S4). Path B is a two-step conversion: trans-ADT raises from the “off” to “B1” state (this process only involves vertical lift of the molecule); then the molecule rotates from the “B1” to “on” state (this process mainly involves lateral rotation; see Figure S4).

Analysis of the adsorption structures of trans-ADT on Cu(111) shows that the adsorption heights of S atoms vary only slightly from the “off” to “on” states. This further confirms the anchoring effect of the S atoms and implies that the different interfacial conductivities at the two


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states arise from the acene core. To understand the conductance switching phenomenon better, we computed the switching ratio (or conductance ratio, Ion/Ioff), which constitutes the central properties of the molecular switch: a large switching ratio is demanded in order to realize the function as a logic and memory elements.3,45 The tunneling current can be written as46 I ∝ Vb ρ(0, Ef )exp(−AΦ1/2 s)

(1)

where Vb is the sample bias voltage with respect to the tip, ρ(0, Ef) represents the local density of states (LDOS) of the scanning area, A = 1.025 Å−1eV−1/2, Φ is the average work function of the STM tip (Φtip) and of the scanning area of the sample (Φsample), and s is the tip-sample distance. The above equation has been extensively used to interpret the relationship between the tunnel current and the sample bias in STM experiments.47-49 At a constant bias voltage, the switching ratio is given by Ion Ioff

=

ρon (0, Ef )exp(−Φ1/2 on son ) ρoff (0, Ef )exp(−Φ1/2 off soff )

(2)

For trans-ADT/Cu(111), the work function Φsample of the “off” and “on” states are almost identical (3.85 and 3.90 eV, respectively) (Table S1). A similar phenomenon has also been reported in olympicenes on Cu(111).26 The tip used in this experiment was W, probably covered with Cu. Note that the work function of the tip apex is here approximated to be the same as that of a flat surface.50 Thereby, we obtain a range of the work function values from 4.5 to 5.8 eV.51 As a consequence, when the vertical position of the STM tip is fixed, the switching ratio of trans-ADT/Cu(111) mainly depends on the magnitudes of the tip-sample distance difference (∆s) of the two states (Figure 3a). Remarkably, when ignoring the difference in ρ(0, Ef ) of the two states, the switching ratio obtained from Equation 2 is close to the experimental data deduced from the I-V curves (3.4-3.7 vs. 3.0).21 The deviation from the experimental values might be attributed to the fact that ρ(0, Ef ) for the “off” state is slightly larger than that for the “on” state,


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leading to smaller switching ratio (see Figure S5). Based on the above analysis, we conclude that the adsorption height change of the acene moieties of the two states provides the dominant contribution to the conductance switching between the “off” and “on” states. a

Tip

ssample-tip

Δs

ssample-tip don

doff

b

2.19 Å 2.49 Å

2.75 Å 2.73 Å

High

Low

Figure 3. (a) Schematic illustration of relative position of the STM tip and the transADT/Cu(111) system at the “off” and “on” states. The sample-tip distance difference (∆s) results from the doff and don. (b) Adsorption geometries and corresponding STM images of the anthracene at the “off” and “on” states above the Cu(111) surface. The two central C atoms (yellow circle in the right panel of plot b) were constrained along the x-y plane at the “on” state during the relaxation. To understand further the role of the acene moieties, adsorption of the anthracene on Cu(111) was also investigated. For this, the starting geometries of anthracene were taken from the “off” and “on” states of trans-ADT on Cu(111) (Figure 3b). The “on” state of anthracene is unstable and converts to the “off” state upon structural relaxations. However, when the X-Y coordinates of the two central C atoms (yellow circle in Figure 3b) of the on-state anthracene were constrained to avoid rotation, an adsorption state similar to the “on” state of trans-ADT was observed. As shown in Figure 3b, the resulting adsorption configurations, adsorption heights, and


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STM images resemble those predicted for trans-ADT on Cu(111). This emphasizes the key role of the acene moieties in the conductance switching of trans-ADT on Cu(111). The analysis above revealed that the switching mechanism of trans-ADT on Cu(111) is governed by the conductance switching from the central carbon rings, accompanied by the anchoring effects from S atoms. In this context, a question naturally arises, whether better molecular switches, with a higher binding energy and larger lateral diffusion barrier, could be designed by substituting side groups. To achieve this goal, we compare two analogues of ADT, trans- ADS and ADF, on Cu(111) (Chart 1). These molecules can be synthesized via selective functionalization on the 3- and 7-positions on 2,6-dimethoxyanthracene.22 Similar to trans-ADT on Cu(111), trans-ADS has a dumb-bell shaped “off” state and a flat shaped “on” state with a rotation of 10.5° (Figure 4a). Furthermore, both Se atoms locate just above the Cu atoms, showing excellent matching degree with the Cu(111) (Table S1; Figure S7). Figure 4b illustrates the potential energy curves of ADS/Cu(111), which shows two minimum at 2.2 and 2.8 Å, respectively, along with different molecular orientations. Interestingly, at adsorption heights higher than 2.90 Å, the off-state position of trans-ADS rotates to the on-state position during geometry relaxations, showing the same rotatory mechanism as trans-ADT on Cu(111). Furthermore, as trans-ADT did, the trans-ADS/Cu(111) system exhibits identical adsorption height difference of the central acene moieties at the two states. Combined with the work function of 3.82 and 3.85 eV at the “off” and “on” states, respectively (Table S1), the switching ratio Ion/Ioff of trans-ADS is identical to that of trans-ADT/Cu(111). Notably, the switching (offto-on) and back-switching (on-to-off) barrier are 0.14 and 0.08 eV, pointing to better bistability than the trans-ADT (Figure S16). The slightly higher rotation barrier of trans-ADS as compared to that of trans-ADT, can be ascribed to the stronger anchoring effect of Se atoms in trans-ADS.


13


Better yet, trans-ADS on Cu(111) presents larger adsorption energy and physisorbed-state lateral diffusion barrier (0.23 eV) because of the stronger anchoring effect of selenium.

a

b

_2.85 _2.90

off on

2.20 Å

on

2.58 Å

2.81 Å

2.56 Å

Eads (eV)

_2.95

off

_3.00 _3.05 _3.10

10.5°

c

2 off _

MODOS (eV 1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

1.70 e

1.15 e

1.77 e

0 2 on

0.33 e

1.83 e

1 1.81 e 0

_4

HOMO-1 HOMO LUMO LUMO+1 0.08 e

_3.15 2.0 2.2 2.4 2.6 2.8 3.0 h (Å)

d

HOMO-1

HOMO

LUMO

LUMO+1

0.08 e

_2

0

2

4

Energy (eV)

Figure 4. Switching of trans-ADS on Cu(111). (a) Adsorption geometries and simulated STM images of trans-ADS at the “off” and “on” states. The STM images were calculated with the bias voltage of 1 V. (b) Potential energy curve of trans-ADS on Cu(111) at the “off” (blue) and “on” (red) state, respectively. (c) Molecular orbital density of states (MODOS) and number of electrons occupied in each molecular orbital of trans-ADS on Cu(111) at the “off” and “on” states, respectively. The zero of energy (gray line) corresponds to the Fermi level. (d) Visualization of the isolated molecular orbitals projected on the HOMO-1, HOMO, LUMO, and LUMO+1 states for trans-ADS. We also study the trans-ADF on Cu(111) and obtain two states therein (Figure S6). Unlike ADS and ADT, for ADF one O atom is located at the top site while the other is located at the bridge site for the “off” state, which is a consequence of lower matching degree (see Table S1 and Figure S7). In light of Equation (2), for trans-ADF on Cu(111) a smaller adsorption height difference (0.21 Å) induces a lower switching ratio than those for trans-ADT and -ADS cases. In addition, the adsorption energy (2.40 eV) and lateral diffusion barrier (0.11 eV) at the


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physisorbed state are significantly lower than that of trans-ADS and -ADT, indicating poor anchoring effect of the O atoms in the furan groups. Thus trans-ADF/Cu(111) is not as good a molecular switch as trans-ADT and -ADS on Cu(111) are. To better understand why anchoring effect exists in ADT and ADS on Cu(111), but not in ADF, we carried out a Bader charge analysis for these three systems.52,53 As shown in Table 1, O atoms in trans-ADF are negatively charged due to its stronger electronegativity than the C atoms, while the S and Se atoms are positively charged. In particular, Se atoms carry larger amount of positive charge than S, which leads to stronger interactions with the metal surface than S atoms. As a consequence, the Se atoms present the best anchoring effect, resulting in the largest lateral diffusion barrier for trans-ADS on Cu(111). Moreover, the different anchoring effect of the above mentioned functional groups can be demonstrated from the adsorption energies of the relative ending functional groups (furan, thiophene, and selenophene) on Cu(111), where selenophene shows the strongest adsorption energy (Figure S8). Thereby, the functional groups extensively control the vertical and lateral stabilities of the molecular switch: the more positive charge the anchoring atoms have, the better vertical and lateral stability the corresponding molecule has. To obtain fundamental understanding of the switching behaviors, the electronic properties of trans-ADS and -ADF on Cu(111) were investigated by analyzing the molecular orbital density of states (MODOS) (Figure 4c). The MODOS was determined by projecting the density of states on the HOMO−1, HOMO, LUMO, and LUMO+1 of the free molecule using the same geometry adsorbate has when adsorbed on the surface.54 For trans-ADS/Cu(111), the frontier orbitals are partly located at the central benzene ring (Figure 4d), being closer to the substrate at the “off” state with respect to the “on” state. As a consequence, the HOMO and LUMO of trans-ADS at


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the “off” state present significant broadening and shifting, in contrast to the weak overlap at the “on” state (Figure 4c). For both states, the substrate interact similarly with the HOMO−1, as a consequence of this orbital being mainly located at the ends of trans-ADS, which have similar adsorption heights at the two states. The MODOS indicates a noticeable difference of the occupied electronic states between the distinct adsorption configurations in trans-ADS. In the “off” state, the HOMO and LUMO are occupied by 1.70 and 1.15 electrons, respectively, thus elevating the HOMO and depressing the LUMO. However, in the “on” state, the charge transfer diminishes, with the HOMO losing 0.17 electrons and LUMO accepting 0.33 electrons. This difference in electron occupation of the two states leads to different coupling between the molecular orbitals and the metal Fermi level, and therefore to different molecule-substrate hybridization and adsorption structures. This explains why positive or negative charge carriers injected into the molecule are able to trigger the conformational change. By contrast, in trans-ADF, the “off” and “on” states show similar hybridizations and charge transfer with the substrate (Figure S10). The MODOS of this molecule indicates that the HOMO and LUMO are further away from the Fermi level than they are for trans-ADS. Based on these findings, it seems reasonable to assume that the HOMO-LUMO energy gap (Eg) of the molecule significantly affects the electronic coupling as well as the interfacial structure. To validate this assumption, we calculated the energy levels of the HOMO and LUMO of isolated trans-ADF, -ADT, and -ADS. The HSE hybrid functional39,40 was used to cure part of the self-interaction error. As shown in Figure 5b, trans-ADF has lower-lying HOMO and higherlying LUMO due to its less effective π-delocalization over the whole π-framework than those of trans-ADS and ADT, agreeing with the photoelectron yield spectroscopy results.22 The ensuing


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larger energy gap, Eg, of trans-ADF means that the frontier molecular orbitals are less disturbed by the substrate, which leads to weaker molecule-substrate hybridizations and interactions. To delve further into this issue, and since the Eg of a molecule is related to its size (the molecules with smaller size exhibit larger Eg22), we investigated how changing the number of central benzene rings in a molecule affects its switching behavior. The molecules selected for investigating this issue included the trans-isomers of BDS, NDS (Chart 1), and TDS. Trans-BDS and -NDS only exhibit on-state characteristics on Cu(111) (Figure 5a and Figure S11). The frontier molecular orbitals are far away from the Fermi level of the Cu(111) surface, leading to slightly disturbed HOMO and LUMO (Figure 5c). For trans-TDS, which has smaller Eg, the HOMO and LUMO are significantly hybridized with the substrate, hindering the formation of the characteristic physisorbed state on Cu(111), i.e., the “on” state of trans-ADS and -ADT cases. As a result, according to Equation 2, the small adsorption height change (0.23 Å, see Figure S11) of the two adsorption structures results in a low switching ratio for trans-TDS on Cu(111). Clearly, the Eg of a molecule substantially determines the magnitude of switching ratio. The analysis above reveals that the off-to-on switching ability is attributed to the moleculesubstrate electronic coupling. The substrate surface should determine in a large extent the stability and structure of the adsorbed molecule. That this is indeed the case is clearly seen in Table S2, where data for trans-ADS on Ag(111), Au(111), Pd(111), Pt(111), Rh(111), and Ir(111) surfaces is presented. Clearly the adsorption energies of trans-ADS are considerably weaker in Ag(111) and Au(111), as compared to that on Cu(111). Further, the molecular orbitals are slightly hybridized with both Ag(111) and Au(111), which results in that only the physisorbed (“on”) states appear (Figure S12 and S14). On the other hand, when trans-ADS is adsorbed on the more active Pd(111), Pt(111), Rh(111), and Ir(111) surfaces, the discrete


17


molecular orbitals are broadened into strong resonances and now only the chemisorbed state exists (Figure S14). In this case, the molecule is covalently bonded to the metal surfaces, leading to drastic molecular deformations (Figure S13). The effect introduced by the substrate is ultimately related to the distribution of the d-band. The wider the d-band is, and the closer it is to the Fermi level, the stronger is the broadening of the molecular orbital (Figure S14). Therefore, a moderate reactivity of the substrate surface is crucial for creating a bistable molecular switch.

a

5

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Size effect on states

off states

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1 0 2

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_

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LUMO

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on

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off TDS

1.04 e 1.77 e 0.08 e

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_4

BDS NDS ADF ADT ADS TDS

_2

0 2 Energy (eV)

off 4

Figure 5. Size effect on the switching performance. (a) Adsorption energies of the trans-isomer of BDS, NDS, ADS, and TDS on Cu(111). (b) HOMO-LUMO energy levels of isolated transisomer of BDS, NDS, ADF, ADT, ADS, and TDS calculated by the HSE hybrid functional. (c) Molecular orbital density of states of trans-BDS, -NDS, -ADS, and -TDS on Cu(111). The gray line shows the Fermi level of Cu(111). Finally, in STM experiments, an electric field is generated within the tunneling junction.55 Taking trans-ADT on Cu(111) as an example, in the process of switching from the “off” to “on” states, a negative sample-tip bias would generate an oriented electric field from the STM tip to the sample,21 which would disturb the distribution of the molecular orbitals2 and perturb the shape of the potential energy curve. In this case, the switching behavior can possibly be


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affected.56,57 This is demonstrated in Figure S15, which shows that cis-ADT tends to adsorb at higher adsorption heights under an external field in the upwards direction normal to the surface. Interestingly, cis-ADT presents the “on” state under a critical electric field of 1.5 V/Å. Moreover, trans-ADT can also be elevated to a physisorbed state via a stronger electric field (2.5 V/Å) due to the deeper potential energy of the “off” state (Figure S15). However, this critical electric field would enlarge and finally break the S-C bond.55 This excludes the possibility of using electric field for the switching of the ADT on Cu(111), in agreement with the experiment findings. In summary, molecular switches based on a reversible transition between a physisorbed and chemisorbed states have a tremendous potential for nanotechnology.20 Recently, such a molecular switch was realized experimentally, and it consisted on the ADT molecule absorbed on Cu(111). We investigated in detail the mechanisms underlying the switching ratio and stability of ADT on Cu(111), and found that the switching ratio is caused by the relative difference in height between the physisorbed and chemisorbed states, whereas the S atoms in ADT anchor the molecule to the substrate, increasing its barrier to diffusion. Based on these findings, we set out to improve ADT/Cu molecular switch, and did so by considering other molecules and other substrates. In this way, we found that trans-ADS, a molecule that contains Se instead of S, when adsorbed on Cu(111) surpassed trans-ADT as a molecular switch: it has a similar switching ratio but higher barrier to diffusion. Along the way we uncovered that the switching performance is a consequence of the molecular charge redistribution and the interfacial electronic coupling. Altogether these findings suggest that to improve the bistability and lateral stability of molecular switches with reversible physisorbed and chemisorbed states, one should


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focus on molecules with a moderate HOMO-LUMO energy gap and anchoring atoms with positive charge, that are deposited on substrates with which they interact moderately. Methods All DFT calculations employed the numeric atom-centered basis set, together with the PBE exchange-correlation functional,58 as implemented in all-electron Fritz Haber Institute ab initio molecular simulations (FHI-aims) package.59,60 The starting geometries were optimized by the PBE+vdWsurf method, which accounts for both the van der Waals interactions and collective response effects. The relativistic effects for Cu, Ag, Au, Rh, Pd, Ir, and Pt atoms were treated by the atomic zero-order regular approximation.61 The “tight” settings including the “tier 2” standard basis set in the FHI-aims code were used for H, C and “tier 1” for S, Se Cu, Ag, Au, Rh, Pd, Ir, and Pt. All atomic positions were relaxed until the maximal force on each atom was smaller than 10−2 eV/Å. The convergence criteria of 10−5 electrons per unit volume for the charge density and 10−6 eV for the total energy of the system were utilized. The two-dimensional Brillouin Zone was sampled with a 3 × 7 × 1 mesh according to the Monkhorst-Pack method.62 To identify the accuracy of switching and lateral diffusion barrier results from the potential energy curve, we also carried out the climbing image nudged elastic band (CI-NEB)41 calculations, as implemented in the Vienna ab initio simulation package (VASP) code.63 This method is widely used for calculating reaction pathways between given initial and final states. In our calculations, seven images were inserted to explore the transition states and barriers. The electric field is realized by introducing an artificial dipole sheet at the center of the simulation cell, following the “perturbation expansion after discretization” formulation of Nunes and Gonze,64 and implemented in VASP. The temperature ensuring the stability of adsorption state was estimated based on Arrhenius expression,65 Γ = νexp(−E/kBT), where Γ, ν, and E are the


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transition rate, the prefactor, and the transition barrier, respectively; kB is the Boltzmann constant and T is the temperature. We used a standard ν value of 1013 s−1 in terms of transition state theory, and Γ values are in the range of 10−2-102 site−1s−1.66 In our calculations, metal surfaces were modelled by five-layer slabs, using a 4 × 8 supercell. The three uppermost metal layers were fully relaxed while the two bottommost layers were constrained at their bulk positions. We used four starting geometries to explore the stable adsorption positions, termed as bridge, top, fcc, and hcp (see Figure S1). Each site was relaxed at adsorption heights of 2.1 and 3.1 Å to probe the existence of the physisorbed and chemisorbed states. The vacuum was set as large as 90 Å in order to avoid the virtual interactions between simulation cells. Associated content Supporting information Work function; matching degree between the adsorbate and substrate; starting geometries; diffusion pathway; energy profiles of switching routine; adsorption structures and adsorption energies of various molecules on metal surfaces; MODOS of different adsorption systems; STM images of adsorption structures under various external electric fields. AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENT We acknowledge support from the NSF of China (51722102, 21773120, 51602155), the Fundamental Research Funds for the Central Universities (30918011340, 30917011201), the NSF of Jiangsu Province (BK20150035), and Jiangsu Key Laboratory of Advanced Micro&Nano Materials and Technology. A portion of this work, interpretation of the data and writing of the manuscript, was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. REFERENCES (1) Irie, M.; Fulcaminato, T.; Matsuda, K.; Kobatake, S. Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators. Chem. Rev. 2014, 114, 1217412277. (2) Lortscher, E. Wiring molecules into circuits. Nat. Nanotechnol. 2013, 8, 381-384. (3) Sun, L.; Diaz-Fernandez, Y. A.; Gschneidtner, T. A.; Westerlund, F.; Lara-Avila, S.; MothPoulsen, K. Single-molecule electronics: from chemical design to functional devices. Chem. Soc. Rev. 2014, 43, 7378-7411. (4) Harzmann, G. D.; Frisenda, R.; van der Zant, H. S. J.; Mayor, M. Single-Molecule Spin Switch Based on Voltage-Triggered Distortion of the Coordination Sphere. Angew. Chem. Int. Edit. 2015, 54, 13425-13430. (5) Aragones, A. C.; Aravena, D.; Cerda, J. I.; Acis-Castillo, Z.; Li, H. P.; Real, J. A.; Sanz, F.; Hihath, J.; Ruiz, E.; Diez-Perez, I. Large Conductance Switching in a Single-Molecule Device through Room Temperature Spin-Dependent Transport. Nano Lett. 2016, 16, 218-226. (6) Zhang, C.; Du, M. H.; Cheng, H. P.; Zhang, X. G.; Roitberg, A. E.; Krause, J. L. Coherent Electron Transport through an Azobenzene Molecule: A Light-Driven Molecular Switch. Phys.


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(61) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. Relativistic total energy using regular approximations. J. Chem. Phys. 1994, 101, 9783-9792. (62) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188. (63) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169. (64) Nunes, R. W.; Gonze, X. Berry-phase treatment of the homogeneous electric field perturbation in insulators. Phys. Rev. B 2001, 63. (65) Laidler, K. J. The development of the Arrhenius equation. J. Chem. Educ. 1984, 61, 494. (66) Nørskov, J. K.; Studt, F.; Abild-Pedersen, F.; Bligaard, T., Fundamental Concepts in Heterogeneous Catalysis; John Wiley & Sons, Inc: Hoboken, NJ, 2014.


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Ir

Se Se

Rh

Se

Pt

Se O

Pd

O Se Se Se Se

Cu physisorption chemisorption

Ag

Bistable molecular switch

Au


stronger interactions with adsorbate

TOC

smaller HOMO-LUMO gap

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Principles of design for substrate-supported molecular switches based

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