Molecular motor crossing the frontier of classical to quantum tunneling motion Samuel Stolza,b, Oliver Gröninga,1, Jan Prinza,b, Harald Bruneb, and Roland Widmera aEmpa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland; and bInstitute of Condensed Matter Physics, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland Edited by Ali Yazdani, Princeton University, Princeton, NJ, and approved May 7, 2020 (received for review October 24, 2019) The reliability by which molecular motor proteins convert un- trigonal surface unit cell (a0 = 6.95Å) forming an equilateral directed energy input into directed motion or transport has inspired triangle of 3.01 Å side length (SI Appendix, Fig. S1 and ref. 25). the design of innumerable artificial molecular motors. We have The local inversion symmetry of this Pd trimer is lifted by co- realized and investigated an artificial molecular motor applying ordination of the six second-layer Ga atoms and furthermore by scanning tunneling microscopy (STM), which consists of a single three Pd atoms in the third layer (Fig. 1 A and B). In the fol- acetylene (C2H2) rotor anchored to a chiral atomic cluster provided lowing we will denote this termination as Pd3. by a PdGa(111) surface that acts as a stator. By breaking spatial On Pd3, acetylene molecules adsorb on top of the Pd trimers inversion symmetry, the stator defines the unique sense of rotation. (26). When imaged by STM at 5 K, they appear as dumbbells While thermally activated motion is nondirected, inelastic electron with lobe-to-lobe separation of about 3 Å in three symmetrically tunneling triggers rotations, where the degree of directionality de- equivalent 120°-rotated orientations (Fig. 1 E–G) between which pends on the magnitude of the STM bias voltage. Below 17 K and they switch quasiinstantaneously (Fig. 1 C and D). Acetylene 30-mV bias voltage, a constant rotation frequency is observed which molecules are firmly anchored to the trimer and usually disso- bears the fundamental characteristics of quantum tunneling. The ciate before being dragged off the trimer by STM-tip manipu- concomitantly high directionality, exceeding 97%, implicates the lation. combination of quantum and nonequilibrium processes in this re- We have followed the rotation events by recording tunneling gime, being the hallmark of macroscopic quantum tunneling. The current time series IT(t) at a fixed tip position (Fig. 1H), in acetylene on PdGa(111) motor therefore pushes molecular machines analogy to the STM investigation of the rotation of chiral PHYSICS to their extreme limits, not just in terms of size, but also regarding butyl–methyl–sulphide on Cu(111) (10). In the latter case, a structural precision, degree of directionality, and cross-over from weak (≤5%) asymmetry in the number of clockwise (CW) nCW classical motion to quantum tunneling. This ultrasmall motor thus n in operando and counterclockwise (CCW) CCW rotations was reported and opens the possibility to investigate effects and origins tentatively attributed to chiral STM tips, as no correlation of the of energy dissipation during tunneling events, and, ultimately, en- directionality with the molecule’s enantiomeric form was found. ergy harvesting at the atomic scales. The IT(t) of Fig. 1H, recorded over Δt = 100 s, exhibits cyclic jump sequences between three levels (...R →R →B →R ...) molecular motor | scanning tunneling microscopy | surface science A B C A with nCCW = 23 jumps in the CCW direction and nCW = 0 in CW, nCCW +nCW resulting in a frequency f = Δt = 0.23 Hz and perfect di- n 1959, Richard Feynman envisioned downscaling of in- nCCW −nCW rectionality dir = 100%pn +n = 100%. Movie SV1 shows a Iformation storage and machines to atomic dimensions (1). CCW CW Both visions were eventually realized: by writing information via Significance positioning single atoms on a nickel surface in 1990 (2), and by devising the first artificial, light-driven molecular machine in 1999 (3). The latter has been inspired by molecular machines in Conversion of undirected energy input into directed motion on molecular scales is the basis for controlled movements in living biological systems (4, 5) and led to the design of countless arti- organisms. In this context, fundamental insights can be ficial molecular machines (6–12). However, most synthetic mo- obtained by investigating artificial molecular machines under lecular machines, although driven by quantum processes, exhibit well-defined conditions. We devised the currently smallest, classical kinetics (13, 14), whereas operation by quantum tun- atomically precise molecular machine, whose rotor (C H ) neling motion is largely elusive. Scanning tunneling microscopy 2 2 consists of just four atoms and whose functioning we have (STM) provides an ideal platform for investigating the dynamics – – tracked employing scanning tunneling microscopy (STM). Un- of atoms and molecules on surfaces (10 12, 15 22). However, like all other reported surface-anchored rotors, ours is charac- few studies were aimed at achieving controlled, STM-tip terized by an extremely high degree of directionality which is position-independent, directional motion that requires breaking independent of STM-tip condition or position, therefore solely of inversion symmetry, which is commonly achieved by adsorbing – defined by the chiral support. Owing to its ultrasmall size, our chiral molecules on achiral surfaces (10 12, 15). We reverse this rotor’s operation crosses the well-established classical to an concept by using the surface of noncentrosymmetric PdGa unanticipated quantum tunneling kinetic regime without loss crystals as chiral stator. This relaxes the geometric constraints on in directionality. the rotor molecule, and allows directed motion even for simple and symmetric molecules such as C2H2. Author contributions: S.S., O.G., J.P., and R.W. designed research; S.S., O.G., J.P., and R.W. The starting point of our study is the creation of a well-defined performed research; S.S., O.G., J.P., and H.B. analyzed data; and S.S., O.G., H.B., and R.W. chiral surface from a noncentrosymmetric single crystal, namely wrote the paper. the intermetallic compound palladium–gallium with 1:1 stoichi- The authors declare no competing interest. ometry (PdGa) exhibiting bulk-terminated chiral surfaces (23). This article is a PNAS Direct Submission. The chiral structure of some of these surfaces manifests itself in Published under the PNAS license. pronounced enantioselective adsorption properties (24). Here 1To whom correspondence may be addressed. Email: [email protected]. we choose the threefold symmetric (111) surface of the PdGa A This article contains supporting information online at https://www.pnas.org/lookup/suppl/ enantiomorph (23). Under appropriate ultrahigh vacuum prep- doi:10.1073/pnas.1918654117/-/DCSupplemental. aration, it terminates by a layer containing three Pd atoms per www.pnas.org/cgi/doi/10.1073/pnas.1918654117 PNAS Latest Articles | 1of5 Downloaded by guest on September 27, 2021 A C D t = t 0 t = t 0 + 60 s Rotation events Scan [-110] direction 5 Å 5 Å [0-11] E F G RA RB RC [0-11] B 5 Å [-110] [0-11] H ] 80 RC RB 65 RA Current [pA 50 5 Å 0 20 40 60 80 100 Time [s] Fig. 1. Acetylene rotation on the PdGa:A(111)Pd3 surface. (A) Sketch of the acetylene (C2H2)onPd3 motor. (B) Atomic structure of the PdGa:A(111)Pd3 surfaces with the PdGa cluster acting as stator highlighted in saturated colors. The C2H2 rotor is depicted in one (Ra) of its three equivalent adsorption configurations Ra,Rb,Rc.InA and B, the top-layered Pd trimers (z = 0) are depicted in bright blue, the second-layer Ga trimers (z =−0.85 Å) in red, and the third-layered single Pd atoms (z =−1.61 Å) in dark blue. (C–G) Constant current STM images of C2H2 adsorbed on the Pd3 surface (T = 5 K; VG = 10 mV; IT = 50 pA). In C two rotating molecules are pointed out, whereas in D, recorded 60 s after C, no molecular rotation is observed. (E–G) STM images of the same acetylene molecule in its three rotational configurations. In E the underlying PdGa stator structure is superposed. (H) Tunneling current time series IT (t) (Δt = 100 s; VG = 25 mV; 1-ms time resolution) measured at the relative position to the C2H2 indicated by the red marker in G. time-lapse series of STM images evidencing the prevailing CCW strongly depends on these parameters. Even though all experi- rotation of the motor. mental data presented in Fig. 1 have been recorded in the TR, we Analyzing the parametric dependence of the rotation fre- first discuss the CR where C2H2 rotations can be selectively quency (Fig. 2 A–C and SI Appendix, Fig. S2) shows that this powered by thermal or electrical excitations. We find the tem- molecular motor operates in two distinct regimes; the tunneling perature dependence of the rotation frequency at low bias regime (TR) where its rotation frequency νT is independent of (Fig. 2A) to follow an Arrhenius characteristic (solid line in | | ΔEB temperature T < 15 K, bias voltage VG < 30 mV, and current Fig. 2A) ν(T)=νT + νAexp( − k T ) [1], with νT = 4.5 Hz, ± B IT < 200 pA, and the classical regime (CR) where the frequency νA = 108.7 2.0Hz (attempt frequency), and ΔEB = 27.5 ± 7.1 meV ABC D 5K 100 100 100 pA, 10 mV 100 pA 12 K 100 100 15 K 17 K 19 K 10 10 10 33 mV 10 1 36 mV 39 mV Frequency [Hz] 1 I 42 mV 1 f ~ T 45 mV 5 1510 20 -40 -20 0 20 40 0 10 403020 50 10 100 1000 Temperature [K] Voltage [mV] Voltage [mV] Current [pA] E F G H -1 0 1 1 10 1‘←3 jump sequence Frequency [Hz] 2‘←1‘ 2→3 3‘←2‘ 1→2 3‘→1 2 Å 2 Å 2 Å time Fig.