Nanoplatelets Bridging a Nanotrench: A New Architecture for Photodetectors with Increased Sensitivity

Abstract

Interparticle charge hopping severely limits the integration of colloidal nanocrystals films for optoelectronic device applications. We propose here to overcome this problem by using high aspect ratio interconnects made of wide electrodes separated by a few tens of namometers, a distance matching the size of a single nanoplatelet. The semiconducting CdSe/CdS nanoplatelet coupling with such electrodes allows an efficient electron-hole pair dissociation despite the large binding energy of the exciton, resulting in optimal photoconductance responsivity. We report the highest responsivity obtained so far for CdSe colloidal material with values reaching kA·W(-1), corresponding to eight decades of enhancement compared to usual micrometer-scaled architectures. In addition, a decrease of 1 order of magnitude of the current noise is observed, revealing the reduced influence of the surface traps on transport. The nanotrench geometry provides top access to ion gel electrolyte gating, allowing for a photoresponsive transistor with 10(4) on/off ratio. A simple analytical model reproduces the device behavior and underlines the key parameters related to its performance.

Strategy to overcome recombination limited photocurrent generation in CsPbX3 nanocrystal arrays

“…This value is closer to 0.5 expected for bimolecular recombination (electron-hole recombination through radiative and non-radiative paths) than to the value of 1 that results from a monomolecular process (trapping) usually observed for metal chalcogenide nanocrystals. 30 This supports the hypothesis of the lower role of surface trapping in PNCs, as expected for a defect-tolerant material in which trap states are away from the band gap. We also investigated the temperature dependence of the current (see Figure 2(c)).…”

“…Along this nanotrench, the two electrodes lie 60 nm apart and have a high aspect ratio (length of 15 µm). Moreover, they present the key advantage of being built with a high fabrication success ratio (>80%) using only two optical lithography steps 30,35 in spite of the sub-wavelength size of the trench. To follow the electric field dependence of the conductance over almost four orders of magnitude, we used µm spaced interdigitated electrodes for the low field range and nanotrench electrodes for the high field range.…”

“…In this nanotrench geometry, the transit time is reduced by a factor of 300 as compared to µm spaced electrodes. 30,36 Because of the n-type nature of these PNCs, the electron will recirculate several times during the hole carrier lifetime, leading to photogating 37 and to the observed enhanced photocurrent. 38 In conclusion, using a combination of DC and time resolved transport measurements, we provide evidence that the photocurrent of PNC arrays is limited by the fast bimolecular recombination of the material.…”

Two-Dimensional Colloidal Nanocrystals

“…8,9 The need to control the shape of the NPs is therefore strong and the growing interest in shape-controlled syntheses of NCs is confirmed by numerous contributions for different applications such as optics (photodetectors, 10,11 lasers, 12 fluorescence [13][14][15] ), electronics, [16][17][18] catalysis, 19 biological sensing 20 and imaging. 21,22 The top-down (chemical or mechanical exfoliation [23][24][25][26] by ion intercalation, ion exchange, sonication 27 or by the "Scotchtape" method 28 ) as well as the bottom-up approaches have so far been explored to synthesize 2D nanostructures with different techniques used such as physical deposition methods (vapor deposition 29 , molecular beam epitaxy (MBE) 30 ), lithography, 31,32 or wet chemical methods.…”

Phototransport in colloidal nanoplatelets array

“…3a. In such nanotrench devices, the photoresponse is dramatically boosted [20] compared to the same material operated in the hopping regime, and responsivity as large as 1 kAW -1 has been demonstrated. This value is the record value obtained for CdSe nanomaterial based devices.…”