Optical manipulation of work function contrasts on metal thin films

Abstract

Work function is a crucial metric in every optoelectronic device to ensure a specific charge transport scheme. However, the number of stable conductive materials available in a given work function range is scant, necessitating work function modulation. As opposed to all the previous chemical methods of work function modulation, we introduce here an alternative approach involving optical modulation. The work function is the minimum energy needed to eject an electron from a solid into vacuum and is known to be light-intensity-independent. A "light intensity dependent" change in work function was observed in metallic thin films coated on a semiconductor. This new phenomenon, contrasting the existing notions on work function, was tested and affirmed with three different systems, namely, Au/n-Si, Pt/n-Si, and W/n-Si. A work function shift of 0.22 eV is achieved in the Pt/n-Si system merely by tuning the illumination intensity from 0 to 18 mW/cm 2 . Continuous tuning of work functions to a specified range is now possible just by tuning the light intensity with a few discrete metals in hand. Moreover, selective illumination creates a work function contrast on the metal film, enabling in-plane charge transport. This throws new light on the design and understanding of the optoelectronic devices. In light of this, we also present a simple photodetector design that is sensitive to illumination direction.

Optical Shading Induces an In‐Plane Potential Gradient in a Semiartificial Photosynthetic System Bringing Photoelectric Synergy

“…The Cu/Si system produced an average surface potential shift of 256 mV and a maximum mode‐to‐mode shift in an individual measurement of 310 mV (Figure S3A, Supporting Information). To our knowledge, light‐induced surface potential shifts reported for metal thin films to date have not previously exceeded 220 mV …”

“…[57] The present work used more abundant and lower cost transition metals, and achieved, in the case of Cu, a higher in-plane potential gradient. [57] The present work used more abundant and lower cost transition metals, and achieved, in the case of Cu, a higher in-plane potential gradient.…”

“…This barrier is typically overcome by photon irradiation enabling electron injection from the semiconductor conduction band to the metal, and Schottky junction solar cells have been developed based on this concept. Under continuous illumination, as photoexcited electrons keep accumulating in the conduction band of the n-Si, those near the barrier with sufficient kinetic energy (known as "hot electrons" [57] ) jump into the metal layer, increasing its surface potential. In "Zone 1" (Figure 7A), the Schottky junction electrode is shaded by a thick, optically opaque membrane multilayer film that prevents photoexcitation of the underlying semiconductor.…”

“…In Zone 2 (Figure B), the lack of a membrane film means that almost 80% of the incident light reaches the Schottky junction inducing photoexcitation of the n‐Si. Under continuous illumination, as photoexcited electrons keep accumulating in the conduction band of the n‐Si, those near the barrier with sufficient kinetic energy (known as “hot electrons”) jump into the metal layer, increasing its surface potential. The continuous nature of the two zones results in a lateral surface potential gradient which drives a flow of electrons from the zone of higher surface potential to the zone with lower surface potential (Figure C).…”

“…The statistical range of the light-induced surface potential shifts from repeat measurements of the three types of Schottky electrode is presented in Figure 3D. [57] The surface potential shift displayed by these Schottky electrodes can manifest as an in-plane potential gradient if a light-intensity gradient is induced on the metal surface. To our knowledge, light-induced surface potential shifts reported for metal thin films to date have not previously exceeded 220 mV.…”