Size and ligand effects are the basis for the novel properties and applications of metallic nanoparticles (NPs) in nanoelectronics, optoelectronics, and biotechnology. This work reports the first observation of enhanced photoelectron emission from metallic Au NPs ligated by alkanethiols. The enhancement is based on a conceptually new mechanism: the AuNP provides electrons while the alkane ligand emits electrons due to its low or negative electron affinity. Moreover, the AuNP-ligand chemical bonding is found to significantly facilitate the transmission of photoexcited electrons from the AuNP to the ligand emitter. Consequently the smooth NP film, which is a typical low-aspect-ratio two-dimensional structure, exhibits strong and stable field emission behavior under photoillumination conditions. The photoenhanced field emission is related to the interband and surface plasmon transitions in AuNPs, and a photoenhancement factor of up to approximately 300 is observed for the AuNP-based field emission. This is highly remarkable because field emission is often based on one-dimensional, high-aspect-ratio nanostructures (e.g., nanotubes and nanowires) with geometrical field enhancement effect. The chemical linkage of electron-supplying AuNP and electron-emitting alkane ligand represents a fundamentally new mechanism for efficient photoexcitation and emission. Being low-temperature/solution processable, and inkjet printable, AuNPs may be a flexible material system for optoelectronic applications such as photodetection and photoenhanced field emission.
Metallic gold (Au) nanoparticles (NPs) are of great interest in nanoelectronics, optics, and biotechnology. [1] In particular, Au NPs exhibit surface plasmon resonance (SPR) [2] that is a result of the collective oscillation of conduction electrons induced by incident electromagnetic radiation. When the size of a metal particle is smaller than the wavelength of incident light, the extinction function is dominated by absorption rather than scattering.[3] Therefore, Au NPs exhibit enhanced photoabsorption in the UV-vis region. Indeed, the SPR characteristic of metallic NPs has been utilized for various applications including molecular sensing [4] and sub-wavelength photonics.[5]Photocurrent generation and photodetection are usually based on semiconductors such as Si and TiO 2 . [6,7] The semiconductor band-gap limits the conduction of intrinsic carriers, while the photoexcited electrons have sufficient energy for transport in the conduction band (CB), thus generating relatively large photocurrent. In a metal, however, there is large population of intrinsic electrons near the Fermi level. When a bias is applied, the number of intrinsic conduction electrons far exceeds that of photoelectrons, such that the intrinsic current is much larger than the photocurrent, rendering the detection of photocurrent impractical in current-voltage (I-V) measurements. This is why there has been almost no report in the literature on the usage of metallic NPs for photocurrent applications.[8] Most works [9][10][11][12] use the SPR behavior of Au NPs to enhance photoabsorption in semiconductors. For example, Au or Ag NPs are dispersed in a semiconductor TiO 2 matrix [9] or on Si [10][11][12] to improve the photon-electron conversion. It is hoped that the enhanced photoabsorption associated with Au NPs will trap more incident light and promote charge separation in the semiconductor. In such a scheme, the photoelectrons are still excited from the semiconductor, while the Au NPs act as an absorption-enhancing agent of the semiconductor. There is only one work briefly reporting the photoconductivity of Au NPs, but neither the detailed conduction mechanism nor the application of photodetection are discussed. [8] In this paper, we report the observation of photoexcitation and photocurrent generation directly from ligated metallic NPs instead of semiconductors. We show that Au-NP passivation by alkanethiol ligands significantly hinders the intrinsic electron conduction, thus increasing the relative ratio of photocurrent to intrinsic current, leading to the photoelectron-dominated current in the NP network. We also discuss the mechanism for photocurrent generation and conduction in the Au NP system, and demonstrate the applicability of metallic Au NPs for photodetection.We first present the observation of photocurrent generation in the Au-NP film prepared according to an early work.[13] A detailed description on the preparation and characterization of Au NPs is provided in the Supporting Information (see SI-i and ii). The right inset of Figure 1...
Photocurrent generation and photodetection are usually based on semiconductor crystals including Si, CdS, and PbS. This work reports the enhanced photoabsorption and photodetection of close-packed metallic Au nanoparticles ͑NPs͒ in the UV-VIS ͑visible͒-NIR ͑near infrared͒ region. Photoabsorption in the UV-VIS regions is associated with the interband transition and surface plasmon resonance of AuNPs, while the enhanced NIR absorption is due to the collective effect of interacting AuNPs in the close-packed network. Consequently, the AuNPs exhibits photodetection behavior in the wavelength range of 300-1500 nm. It is proposed that the inter-AuNP photoejection and delocalization of electron-hole pairs changes the carrier lifetime and transit dynamics in favor of photocarrier conduction, thus significantly facilitating photocurrent generation in the metallic AuNP close-pack. Moreover, due to the power-law conduction mechanism in AuNP networks, the quantum yield of AuNPs can be tuned from 10 −6 to 10 −1 photoelectron/photon by increasing the bias voltage from 0 to 5 V. The AuNP quantum yield of 10 −1 at 5 V is as high as that of commercial Si photodetectors at 0 V, and this demonstrates the immediate applicability of AuNPs in photodetection. In view of the compatibility of AuNPs with wet-chemistry and inkjet printing processes at low temperatures, metallic AuNPs may provide a convenient alternative to semiconductor crystals in photodetection and perhaps photovoltaic applications.
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