Browsing by Author "Razeghi, Mohammadali"

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Open Access
Photocurrent generation in low dimensional nanomaterials
(2022-11) Razeghi, Mohammadali
This thesis focuses on crucial issue on the understanding the underlying mechanisms of photoresponse in low-dimensional nanomaterials. As the size goes down to the micro and nano level, fine features and induced inhomogeneities like strain, thickness variation, substrate, and junctions become influential in determining plausible effects that can explain and control the light-matter interactions in an optoelectronic device. To develop a better understanding of the fundamental physical characteristics of nanomaterials and optimize thermal and electrical transport in nanomaterial devices, microscopic investigation at a single crystal level is required. In this thesis, I investigated photocurrent generation in two extreme cases: metallic silver nanowire (Ag NW) and semiconducting multilayer molybdenum disulfide (MoS2) using scanning photocurrent microscopy (SPCM). SPCM provides spatial mapping of photoresponse along with corresponding reflected light intensity with a few hundred nanometer resolution. Two terminal devices of Ag and Ag network devices are made by drop-casting NW and placing indium as metal contacts. The SPCM maps show that the NW- NW junctions and NW-contacts interface locally enhance the plasmonic field and act as hot spots. The increased temperature at hot spots is enough to modulate the resistance and results in a photo-bolometric response under the bias voltage. To further enhance the photo-bolometric effect, we decorated the nanowires with plasmonic Ag nanoparticles. The nanoparticles increase the number of hot spots and strengthen light coupling into plasmons. We also attributed zero bias response to the photothermoelectric effect. The photocurrent is generated by the Seebeck coefficient difference caused by nanogaps and nonuniformities in the geometry along the Ag NW. The second part of this thesis describes photocurrent generation by substrate engineering of a few-layer MoS2. To partially suspend a crystal, a flake of MoS2 is exfoliated and then transferred on a substrate with rectangular or circular holes. We observed photocurrent generation from the junction of the supported and suspended parts. Substrate effects like induced doping play an essential role in determining the properties of two-dimensional materials. Our investigations show that the Seebeck coefficient of the suspended part is changed due to isolation from the substrate. The difference in the Seebeck coefficient of suspended and supported regions forms a thermoelectric junction. We also investigated the impact of carrier type and concentration on photocurrent generation by gating experiments.
Open Access
Plasmon-enhanced photoresponse of single silver nanowires and their network devices
(Royal Society of Chemistry, 2022-02-11) Razeghi, Mohammadali; Üstünçelik, Merve; Shabani, Farzan; Demir, Hilmi Volkan; Kasırga, T. Serkan
The photo-bolometric effect is critically important in optoelectronic structures and devices employing metallic electrodes with nanoscale features due to heating caused by the plasmonic field enhancement. One peculiar case is individual silver nanowires (Ag NWs) and their networks. Ag NW-networks exhibit excellent thermal, electrical, and mechanical properties, providing a simple yet reliable alternative to common flexible transparent electrode materials used in optoelectronic devices. To date, there have been no reports on the photoresponse of Ag NWs. In this study, we show that single Ag NWs and networks of such Ag NWs possess a significant, intrinsic photoresponse, thanks to the photo-bolometric effect, as directly observed and measured using scanning photocurrent microscopy. Surface plasmon polaritons (SPPs) created at the contact metals or plasmons created at the nanowire-metal structures cause heating at the junctions where a plasmonic field enhancement is possible. The local heating of the Ag NWs results in negative photoconductance due to the bolometric effect. Here an open-circuit response due to the plasmon-enhanced Seebeck effect was recorded at the NW-metal contact junctions. The SPP-assisted bolometric effect is found to be further enhanced by decorating the Ag NWs with Ag nanoparticles. These observations are relevant to the use of metallic nanowires in plasmonic applications in particular and in optoelectronics in general. Our findings may pave the path for plasmonics-enabled sensing without spectroscopic detection.
Open Access
Single-material MoS2 thermoelectric junction enabled by substrate engineering
(Nature Research, 2023-05-26) Razeghi, Mohammadali; Spiece, J.; Oğuz, Oğuzhan; Pehlivanoğlu, Doruk; Huang, Y.; Sheraz, Ali; Başçı, U.; Dobson, P. S.; Weaver, J. M. R.; Gehring, P.; Kasırga, Talip Serkan
To realize a thermoelectric power generator, typically, a junction between two materials with different Seebeck coefficients needs to be fabricated. Such differences in Seebeck coefficients can be induced by doping, which renders it difficult when working with two-dimensional (2d) materials. However, doping is not the only way to modulate the Seebeck coefficient of a 2d material. Substrate-altered electron–phonon scattering mechanisms can also be used to this end. Here, we employ the substrate effects to form a thermoelectric junction in ultrathin, few-layer MoS2 films. We investigated the junctions with a combination of scanning photocurrent microscopy and scanning thermal microscopy. This allows us to reveal that thermoelectric junctions form across the substrate-engineered parts. We attribute this to a gating effect induced by interfacial charges in combination with alterations in the electron–phonon scattering mechanisms. This work demonstrates that substrate engineering is a promising strategy for developing future compact thin-film thermoelectric power generators. © 2023, The Author(s).