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Interfacial and Geometric Effects on the Physics of Quantum Dot/Graphene Nanohybrid Photodetectors for Broadband and Flexible Imagers /

Record Type:	Electronic resources : Monograph/item
Title/Author:	Interfacial and Geometric Effects on the Physics of Quantum Dot/Graphene Nanohybrid Photodetectors for Broadband and Flexible Imagers // Andrew Shultz.
Author:	Shultz, Andrew,
Description:	1 electronic resource (102 pages)
Notes:	Source: Dissertations Abstracts International, Volume: 86-02, Section: B.
Contained By:	Dissertations Abstracts International86-02B.
Subject:	Condensed matter physics. -
Online resource:	https://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=31300901
ISBN:	9798384016922

Interfacial and Geometric Effects on the Physics of Quantum Dot/Graphene Nanohybrid Photodetectors for Broadband and Flexible Imagers /
Shultz, Andrew,

Interfacial and Geometric Effects on the Physics of Quantum Dot/Graphene Nanohybrid Photodetectors for Broadband and Flexible Imagers /Andrew Shultz. - 1 electronic resource (102 pages)

Source: Dissertations Abstracts International, Volume: 86-02, Section: B.

Conventional photodetectors have been extensively developed and have become highly matured. Limitations imposed by the external quantum efficiency (EQE), noise, and mechanical flexibility have created a roadblock for further improvements in detector sensitivity. Constraints on photodetector performance are imposed by the EQE, which conventional designs typically cannot achieve an EQE higher than 100%. Hybrid structures, ones that separate photocarrier generation and charge transport into different materials, have proven that an EQE much higher than 100% is possible. The noise issue is most severe for photodetection in the infrared regime, requiring semiconductors with small bandgaps, which are prone to free carrier generation at ambient temperatures in dark. Cooling is typically employed to reduce the dark current via reduction of phonon energy. Negating the need for cooling is one focus for next generation infrared detectors. Lastly, mechanically flexible devices have become increasingly desirable for wearable devices and the potential to greatly simplify imaging systems by reduced need for bulky and expensive optical components for image focusing. Conventional photodetector paradigms are based on rigid materials, substrates, and readout integrated circuits which prevent flexibility. Nanohybrid photodetectors are quantum sensors promising next generation photodetectors by means of overcoming the present EQE, dark current, and flexibility limitations. Nanohybrid photodetectors seek to overcome dark current limitations by means of special properties inherent to quantumly confined materials. Quantum dots (QD) are zero dimensional semiconductors that exhibit suppressed phonon scattering and bandgap tunability not found in the bulk form. Similarly, graphene is a 2-dimensional material boasting exceptional carrier mobility. Marriage of the 2 materials to form a QD/graphene nanohybrid aims to provide a detector with the unique advantages of both materials to enable high EQE, low dark current, and flexibility. Quantum confinement in QDs enables suppressed phonon scattering, leading to reduced dark current, but suffer from poor carrier mobility, typically on the order 10-2 cm2V-1s-1 for PbS QDs. Monolayer graphene on the other hand displays an exceptional carrier mobility typically on the order of 104 cm2V-1s-1, owing to massless carriers emerging from quantum confinement in graphene. Equilibration of QD and graphene Fermi levels at the interfacial contact forms a favorable electric field that promotes exciton dissociation in the QDs and carrier transfer to the graphene which can in turn circulate in external readout electronics many times before recombination. A photogenerated charge traversing an external circuit more than once results in photoconductive gain, this can be thought of as the number of charges collected at the anode per absorbed photon, which translates to an EQE higher than 100%. Defects, surface states, and/or a poor interface on the QDs may prevent charge transfer, leading to an EQE as low as 0%. Detector geometry also plays a role as the photoconductive gain, and hence EQE, is dependent on the ratio of recombination time to transit time, thus warranting an investigation of the geometric effects. Furthermore, noise in the QD/graphene nanohybrid is dominated by 1/f noise in graphene, allowing a bypass to the thermal noise limitation for infrared wavelengths. Lastly, both QDs and graphene have a low form factor that permits flexibility without significant damage. This thesis work probed the physics of the nanohybrid to better understand performance constraints. First, QDs are solution synthesized, which presents a challenge for ensuring a clean, charge trap free, interface between QD-QD as well as QD-graphene. Similarly, the graphene transfer process can introduce insulating residues on the graphene surface. These interfaces were subjected to an intensive study to develop an understanding of the limiting factors preventing efficient charge transfer, for QDs this amounts to passivation of surface states via conducting ligands to protect from undesired molecular adsorption whereby trap states could form and for graphene, the removal of polar molecules as well as contaminants to ensure a clean surface. Protection from ambient molecules requires capping the QDs with insulating ligands immediately after synthesis, which are in turn replaced during device fabrication. A layer-by-layer approach was developed to maximize the exchange of insulating ligands with conducting ligands, using the photoresponse, response speed, and noise as indications for the underlying physical arrangement. The optimized interfaces in the QD/Gr nanohybrids would allow probing of the photoconductive gain dependence on geometric factors predicted theoretically. This thesis research also experimentally tested the applicability of these theories to the QD/graphene nanohybrid quantum devices, finding a good agreement with predictions. The graphene 1/f noise dependence on geometric factors was also investigated, for which predictions and data were not previously available, providing valuable insight into the underlying noise physics of graphene which differs from conventional semiconductor noise in origin. The final topic of this thesis work focused on the development of an imaging array of QD/graphene nanohybrids as a demonstration of practical applicability. A flexible array was developed in parallel with a rigid array to allow performance comparison. Two imaging techniques were employed, full array imaging and single pixel image scanning, both providing high quality images with rigid and flexible detector arrays. Finally, bending tests were conducted to demonstrate continued functionality of the flexible array and effects on photoresponse.

English

ISBN: 9798384016922Subjects--Topical Terms:

3173567
Condensed matter physics.
Subjects--Index Terms:

Graphene

Interfacial and Geometric Effects on the Physics of Quantum Dot/Graphene Nanohybrid Photodetectors for Broadband and Flexible Imagers /
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520 $a Conventional photodetectors have been extensively developed and have become highly matured. Limitations imposed by the external quantum efficiency (EQE), noise, and mechanical flexibility have created a roadblock for further improvements in detector sensitivity. Constraints on photodetector performance are imposed by the EQE, which conventional designs typically cannot achieve an EQE higher than 100%. Hybrid structures, ones that separate photocarrier generation and charge transport into different materials, have proven that an EQE much higher than 100% is possible. The noise issue is most severe for photodetection in the infrared regime, requiring semiconductors with small bandgaps, which are prone to free carrier generation at ambient temperatures in dark. Cooling is typically employed to reduce the dark current via reduction of phonon energy. Negating the need for cooling is one focus for next generation infrared detectors. Lastly, mechanically flexible devices have become increasingly desirable for wearable devices and the potential to greatly simplify imaging systems by reduced need for bulky and expensive optical components for image focusing. Conventional photodetector paradigms are based on rigid materials, substrates, and readout integrated circuits which prevent flexibility. Nanohybrid photodetectors are quantum sensors promising next generation photodetectors by means of overcoming the present EQE, dark current, and flexibility limitations. Nanohybrid photodetectors seek to overcome dark current limitations by means of special properties inherent to quantumly confined materials. Quantum dots (QD) are zero dimensional semiconductors that exhibit suppressed phonon scattering and bandgap tunability not found in the bulk form. Similarly, graphene is a 2-dimensional material boasting exceptional carrier mobility. Marriage of the 2 materials to form a QD/graphene nanohybrid aims to provide a detector with the unique advantages of both materials to enable high EQE, low dark current, and flexibility. Quantum confinement in QDs enables suppressed phonon scattering, leading to reduced dark current, but suffer from poor carrier mobility, typically on the order 10-2 cm2V-1s-1 for PbS QDs. Monolayer graphene on the other hand displays an exceptional carrier mobility typically on the order of 104 cm2V-1s-1, owing to massless carriers emerging from quantum confinement in graphene. Equilibration of QD and graphene Fermi levels at the interfacial contact forms a favorable electric field that promotes exciton dissociation in the QDs and carrier transfer to the graphene which can in turn circulate in external readout electronics many times before recombination. A photogenerated charge traversing an external circuit more than once results in photoconductive gain, this can be thought of as the number of charges collected at the anode per absorbed photon, which translates to an EQE higher than 100%. Defects, surface states, and/or a poor interface on the QDs may prevent charge transfer, leading to an EQE as low as 0%. Detector geometry also plays a role as the photoconductive gain, and hence EQE, is dependent on the ratio of recombination time to transit time, thus warranting an investigation of the geometric effects. Furthermore, noise in the QD/graphene nanohybrid is dominated by 1/f noise in graphene, allowing a bypass to the thermal noise limitation for infrared wavelengths. Lastly, both QDs and graphene have a low form factor that permits flexibility without significant damage. This thesis work probed the physics of the nanohybrid to better understand performance constraints. First, QDs are solution synthesized, which presents a challenge for ensuring a clean, charge trap free, interface between QD-QD as well as QD-graphene. Similarly, the graphene transfer process can introduce insulating residues on the graphene surface. These interfaces were subjected to an intensive study to develop an understanding of the limiting factors preventing efficient charge transfer, for QDs this amounts to passivation of surface states via conducting ligands to protect from undesired molecular adsorption whereby trap states could form and for graphene, the removal of polar molecules as well as contaminants to ensure a clean surface. Protection from ambient molecules requires capping the QDs with insulating ligands immediately after synthesis, which are in turn replaced during device fabrication. A layer-by-layer approach was developed to maximize the exchange of insulating ligands with conducting ligands, using the photoresponse, response speed, and noise as indications for the underlying physical arrangement. The optimized interfaces in the QD/Gr nanohybrids would allow probing of the photoconductive gain dependence on geometric factors predicted theoretically. This thesis research also experimentally tested the applicability of these theories to the QD/graphene nanohybrid quantum devices, finding a good agreement with predictions. The graphene 1/f noise dependence on geometric factors was also investigated, for which predictions and data were not previously available, providing valuable insight into the underlying noise physics of graphene which differs from conventional semiconductor noise in origin. The final topic of this thesis work focused on the development of an imaging array of QD/graphene nanohybrids as a demonstration of practical applicability. A flexible array was developed in parallel with a rigid array to allow performance comparison. Two imaging techniques were employed, full array imaging and single pixel image scanning, both providing high quality images with rigid and flexible detector arrays. Finally, bending tests were conducted to demonstrate continued functionality of the flexible array and effects on photoresponse.
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653 $a Geometric effects
653 $a Photoconductive gain
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710 2 $a University of Kansas. $b Physics & Astronomy. $e degree granting institution. $3 3822614
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