Enhanced near-Infrared Photoresponse from Nanoscale Ag-Au Alloyed Films

Abstract

Alloying of metals provides a vast parameter space for tuning of material, chemical, and mechanical properties, impacting disciplines ranging from photonics and catalysis to aerospace. From an optical point-of-view, pure thin metal films yield enhanced light absorption due to their cavity effects. However, an ideal metal–semiconductor photodetector requires not only high absorption, but also long hot carrier attenuation lengths in order to efficiently collect excited carriers. Here we demonstrate that Ag-Au alloys provide an ideal model system for controlling the optical and electrical responses in nanoscale thin metal films for hot carrier photodetectors with improved performance. While pure Ag and Au have long hot carrier attenuation lengths >20 nm, their optical absorption is insufficient for high efficiency devices. Instead, we find that alloying Ag and Au enhances the absorption by ∼50% while maintaining attenuation lengths >15 nm, currently limited by grain boundary scattering, although the electron attenuation length of pure Au outperforms pure Ag as well as all of the alloys investigated here. Further, our density functional theory analysis shows that the addition of small amounts of Au to the Ag lattice significantly enhances the hot hole generation rate. Combined, these findings suggest a route to high efficiency hot carrier devices based on metallic alloying with potential applications ranging from photodetectors and sensors to improved catalytic materials.

Emergent Opportunities with Metallic Alloys: From Material Design to Optical Devices

“…One of the most common examples are the superabsorbers, [19] which will be discussed in Section 4.1. More recently, the benefits of using ultrathin metallic films to increase light absorption in the NIR range of the electromagnetic spectrum have been demonstrated in sys tems ranging from hotcarrier devices composed by Si/ultrathin layers of metals and alloys [27,68] to indexnearzero substrate and ultrathin metals. [69] In these devices, the large optical absorp tion results from the combined effect of large optical attenua tion within the lossy thinfilm material (e.g., semiconductors or metals) and the nontrivial interface phase shifts.…”

“…In the past, there have been various reports on the direct calculation of the band structure of alloys using density func tional theory (DFT). [52,65] As a very recent example, Krayer et al [68] have shown explicitly how each metal contributes to the overall band structure of the Ag-Au alloys. As presented in Figure 3h, Au dominates most of the highenergy states in the alloy's dband, whereas comparable contributions from Au and Ag are observed for other states across all critical points of the Brillouin zone.…”

“…Alloying can also be used to increase light absorption within the metal, while still maintaining long carrier attenuation lengths, as shown by Krayer et al with Ag-Au alloys. [68] With known hot carrier energy profiles, one can choose proper doping types and concentrations in the semicon ductors to form Schottky junctions with adequate barrier height for efficient current generation.…”

“…with Ag–Au alloys. [ 68 ] With known hot carrier energy profiles, one can choose proper doping types and concentrations in the semiconductors to form Schottky junctions with adequate barrier height for efficient current generation.…”

Efficient hot carrier dynamics in near-infrared photocatalytic metals

“…These carriers are generated in the mid-infrared and near-infrared range, and subsequent engineering of the electronic and optical properties by alloying them with other noble metals has emerged as a route for the efficient generation of hot carriers as this procedure brings the positions of the d-band closer to the Fermi level, E F [28][29][30][31]. Consequently it reduces the interband energy threshold, permitting the efficient generation of long-lived hot carriers in the infrared and near-infrared region [32,33].…”