Fabrication of Near-Infrared Photodetectors with Ag-Cu-in-Te Nanorod Films as an Active Layer
Authors: Kubota Sho , Muto Chisato , Kameyama Tatsuya , Kuwabata Susumu , Torimoto Tsukasa 
Published: December 2024

Semiconductor nanocrystals (NCs) have been intensively investigated for the application to photovoltaics, light-emitting diodes, and biological markers, because of their tunable light absorption and excellent light emission properties. In our previous paper[1], we reported the synthesis of near-IR-responsive AgInTe2 NCs by a colloidal method, in which the obtained NCs showed a relatively sharp photoluminescence (PL) peak at 1100 nm with a quantum yield of 47%. In addition, solid solution NCs of ZnTe and AgInTe2 were found to have tunable energy gap (Eg
) in the wide near-IR region from 1.2 to 1.6 eV by controlling their chemical composition[2]. Here, we explore CuInTe2, an I-III-VI semiconductor with the Eg of 0.86 eV, aiming to achieve Eg control in a narrower range from 0.86 to 1.2 eV by forming solid solutions with AgInTe2. This paper reports the solution-phase synthesis of Ag(1-x)Cu
x
InTe2 (ACITe) solid solution NCs and evaluates their optical detection capabilities for near-IR light. We demonstrate that the particle size and composition of ACITe NCs can be modified to fine tune their photochemical properties.

Metal acetates of Ag, Cu, and In were added to 1-dodecanethiol in a test tube and stirred at room temperature under a nitrogen atmosphere. Trioctylphosphine telluride (TOP-Te) was added to the mixture and heated at 180°C for 10 min with vigorous stirring. The resulting ACITe NCs were isolated and purified, followed by dissolving in octane. A seed-mediated crystal growth method was used to control the particle size: The thus-obtained ACITe NCs were added as a seed and further heated with 1-dodecanethiol containing corresponding metal acetates and TOP-Te. The size of NCs was controlled by changing the amount of ACITe seeds added.

ACITe NCs were rod-shaped particles, regardless of their chemical composition, Cu/(Ag+Cu) (= x). The XRD pattern of NCs prepared with x= 0 was attributed to hexagonal AgInTe2 crystal structure. As the Cu fraction increased, i.e. the x value increased, each peak in the XRD pattern of ACITe NCs was shifted to a higher angle, approaching that of tetragonal CuInTe2 crystal structure. These results indicated that the thus-obtained NCs were composed of a solid solution between AgInTe2 and CuInTe2, the composition of which was controlled by the Cu/Au ratio in the precursors. With the elongation of the rod NCs, the XRD peak around 25° corresponding to the hexagonal (002) plane became narrowed, indicating that ACITe NCs were grown along with the c-axis of hexagonal crystal structure. The onset wavelength of the absorption spectra of ACITe NCs gradually shifted from 1070 nm (x = 0, AgInTe2) to 1200 nm (x=1, CuInTe2) with increasing the x value, indicating that E
g of ACITe NCs was tuned in the near-IR region by controlling their composition.

The obtained ACITe NCs (x= 0.5) with average rod lengths of 13 nm, 23 nm, and 27 nm (denoted as ACITe(13 nm), ACITe(23 nm), and ACITe(27 nm), respectively) were used to fabricate a photodetector, in which the QDs were spin-coated on a comb-shaped electrode. We evaluated the photoresponse at the applied voltage of +5.0 V under a dark condition or under the irradiation of near-IR light (880 nm) with various intensities. Regardless of the light intensity, the devices prepared with ACITe(23 nm) and ACITe(27 nm) exhibited approximately 5-7 times higher sensitivity compared to that with ACITe(13 nm), indicating that the crystal morphology of semiconductor NCs significantly affected their performance of near-IR light detection. Moreover, the photosensitivity at the irradiation intensity of 20 µW cm-2 was 20.3 A W-1 for the device with ACITe(27 nm), which was higher than that reported for a photodetector fabricated with conventional near-IR-responsive PbSe NCs (0.66 A W-1; 1064 nm, 10 µW cm-2) [3].

References

(1) T. Kameyama, et al., Nanoscale, 8, 5435 (2016). (2) T. Kameyama, et al., J. Mater. Chem. C, 6, 2034 (2018). (3) A. A. Babaev, et al., Nanomaterials, 13, 3051 (2023).