The reliability by which molecular motor proteins convert undirected energy input into directed motion or transport has inspired the design of innumerable artificial molecular motors. We have realized and investigated an artificial molecular motor applying scanning tunneling microscopy (STM), which consists of a single acetylene (C₂H₂) rotor anchored to a chiral atomic cluster provided by a PdGa(111) surface that acts as a stator. By breaking spatial inversion symmetry, the stator defines the unique sense of rotation. While thermally activated motion is nondirected, inelastic electron tunneling triggers rotations, where the degree of directionality depends on the magnitude of the STM bias voltage. Below 17 K and 30-mV bias voltage, a constant rotation frequency is observed which bears the fundamental characteristics of quantum tunneling. The concomitantly high directionality, exceeding 97%, implicates the combination of quantum and nonequilibrium processes in this regime, being the hallmark of macroscopic quantum tunneling. The acetylene on PdGa(111) motor therefore pushes molecular machines to their extreme limits, not just in terms of size, but also regarding structural precision, degree of directionality, and cross-over from classical motion to quantum tunneling. This ultrasmall motor thus opens the possibility to investigate in operando effects and origins of energy dissipation during tunneling events, and, ultimately, energy harvesting at the atomic scales.

分子马达蛋白将无方向的能量输入转换为定向运动或运输的可靠性，激发了无数人工分子马达的设计。我们已经意识到并研究了一种应用扫描隧道显微镜（STM）的人工分子马达，该马达由单个乙炔（C ₂ H ₂）转子锚定到由用作定子的PdGa（111）表面提供的手性原子簇上。通过打破空间反转对称性，定子确定了独特的旋转方向。尽管热激活运动是非定向的，但非弹性电子隧穿会触发旋转，其方向性取决于STM偏置电压的大小。在17 K和30 mV偏置电压以下，观察到恒定的旋转频率，该频率具有量子隧穿的基本特性。随之而来的高方向性（超过97％）暗示了在这种情况下量子和非平衡过程的结合，这是宏观量子隧穿的标志。因此，PdGa（111）电动机上的乙炔将分子机器推向极限，不仅仅是大小上的限制，而且还涉及结构精度，方向性程度以及从经典运动到量子隧穿的交叉。因此，这种超小型电动机为研究提供了可能性在隧道效应期间的操作效应和能量耗散的起源，以及最终在原子尺度上的能量收集。