Article Cite This: J. Phys. Chem. C 2018, 122, 3176−3181
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Shell Thickness-Dependent Tunable Threshold Voltage Single Quantum Dot Rectification Diode Gopal Sankar Kenath,† Rekha Mahadevu,§ Anand Sharma,† Vinod K. Gangwar,‡ Sandip Chaterjee,‡ Anshu Pandey,§ and Bhola N. Pal*,† †
School of Materials Science and Technology and ‡Department of Physics, Indian Institute of Technology (BHU), Varanasi 221005, India § Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India S Supporting Information *
ABSTRACT: Ambient atmosphere single colloidal quantum dot (QD) rectifying diode with tunable threshold voltage has been fabricated by using a type-II heterojunction core/shell structure with a device geometry ITO/ZnO/QDs. Specifically, in our work we have used ZnTe/CdS core/shell QDs in which hole wave function strongly confined to the core, whereas the lowest-lying conduction band state resides in the shell. Current−voltage (I−V) characterization of this device has been done using an ambient atmosphere scanning tunneling microscope. The scanning tunneling spectra (STS) shows high rectification with a ratio of 103. The rectification is found to arise because of the bias-dependent band alignment of ZnO/ QDs heterojunction and the effect of shell of each QD that presents a barrier for hole tunneling into the substrate. This barrier is overcome by the externally applied bias. This mechanism is distinct from the rectification observed in conventional p−n junction diodes. In particular, we find that even for QDs with optical band gaps of ∼1 eV, the threshold voltage may be tuned from 1 to 3 V by regulating shell thickness.
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INTRODUCTION The constant need for smaller and faster devices has pushed the frontiers of research into the nanoelectronics regime.1 Nanoelectronic devices that employ materials in the nanometer range can achieve such a goal.2−4 Studies using nanomaterials such a graphene, carbon nanotubes, and organic molecules have risen in prominence over the past few decades.4−11 Of late, single nanocrystal interface rectification behavior has been observed in different types of heterojunction including 2D semiconductor, doped semiconductor, and aligned magnetic domains in p- and n-type ferromagnetic.12−14 In addition to these inorganic nanostructure approaches, organic molecule-based nanoelectronics devices have received considerable attention because of their versatile properties such as mechanical flexibility, low fabrication cost, and smaller dimensions.15 These devices can also be tuned as per device requirement by altering the structure of the molecules used. Several studies have been conducted on the fabrication of switches and rectifiers using a single organic molecule.8,9,11,16,17 A common theme in these studies is the use of a scanning tunneling microscope (STM) to characterize the electronic properties and the charge transport across these devices.10,18,19 QDs offer a viable alternative to already successful molecular devices. Like organic molecules, they exhibit properties such as monodispersibilty, self-assembly, intrinsic quantum mechanical behavior, low-cost solution processability, and so on.20 In addition to these properties, QDs are far superior considering © 2018 American Chemical Society
their environmental stability, photochemical stability, narrow emission bandwidth, internal quantum yield, and crystalline nature.21−23 The optoelectronic characteristics of the QDs can be tuned by altering the size, doping concentration, or shell thickness over the core QDs.22,24,25 Therefore, there are significant benefits in fabricating single dot devices in the future.26−28 In our present study, we have developed an ambient atmosphere single QD rectifying diode with a tunable threshold voltage. The rectification ratio of this device is ∼103, which is much higher than the recently published single-molecule organic diode.29 A type-II core/shell heterostructure QD has been used for the fabrication of single dot device, where the threshold voltage was controlled as per shell thickness. In such kind of core/shell structure, hole wave function is strongly localized within the core, whereas electron wave function is strongly localized within the shell only.30,31 Taking the advantage of this property, we have fabricated shell-thicknessdependent tunable threshold voltage single QD rectifying diode. In particular, in our study, QDs with a ZnTe core and a CdS shell were used to fabricate the rectifiers whose threshold voltage was tuned from 0.9 to 2.65 V. It is interesting to note that the threshold voltage follows the opposite trend to the QD Received: December 31, 2017 Revised: January 15, 2018 Published: January 16, 2018 3176
DOI: 10.1021/acs.jpcc.7b12837 J. Phys. Chem. C 2018, 122, 3176−3181
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The Journal of Physical Chemistry C
Figure 1. (a) Absorption spectra of ZnTe core (band gap of the ZnTe core is ∼2.77 eV). (b) Absorption spectra of three different shell thickness of ZnTe/CdS core−shell QDs used for our measurements (CdS shell thickness increases from right to left). (c) Powder XRD patterns of ZnTe/CdS samples used for rectification measurements (shell thickness increases from bottom to top). As shell thickness increases, peaks corresponding to ZnTe start diminishing, while a peak that corresponds to CdS starts appearing. (d) TEM image of ZnTe core with an average particle size 3.5 nm and (e) HRTEM images of ZnTe/CdS QDs with average particle size 7 nm.
(blue line). This clearly indicates that there is very thin shell of CdS on top ZnTe core. As the shell thickness increases, the peak corresponding to CdS patterns starts appearing (green line and red line, Figure 1c). This indicates the thick shell of CdS on ZnTe core. Figure 1d shows the transmission electron micrograph (TEM) of ZnTe core with average particle size of 3.5 nm, whereas Figure 1e shows the high-resolution transmission electron micrograph (HRTEM) of ZnTe/CdS with average particle size 7 nm. This HRTEM image of the quantum dot clearly indicates the crystalline nature of individual dot. The ZnO, on the contrary, was synthesized via sol−gel route using zinc acetate dihydrate, 2-methoxyethanol, and monoethanolamine (MEA) as precursor. In the beginning of this process, zinc acetate dihydrate (500 mM) solution was continuously stirred for 30 min; then, the solution was filtered using a syringe filter (PVDF membrane, 0.22 μm) to remove large particles. The precursor solution of 500 mM concentration was deposited on clean indium tin oxide (ITO)-coated glass substrate by spin coating with a spinning speed 5000 rpm, followed by an ambient environment annealing at 350 °C for 30 min. This annealing step forms a crystalline thin film of ZnO. A QD solution of 5 mg/mL was then spin-coated on the ZnO layer with a spinning speed of 1500 rpm. The resulting ITO/ZnO/QD film was taken for STM studies.
band gap, with the QDs having thickest CdS shells and narrowest band gaps giving rise to the highest thresholds. This is contrary to the behavior of conventional p−n junction diodes, where the threshold voltage is essentially limited by QD band gap.
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EXPERIMENTAL SECTION Single QD rectification devices were fabricated on top of an ITO glass substrate overcoated with a ZnO layer. Synthesis of ZnTe/CdS core/shell QDs was done by following a previously reported procedure.32 A detailed synthetic procedure is provided in the Supporting Information. In brief, there a twostep procedure is employed. First, elemental tellurium is reduced to telluride using sodium borohydride. This reduction step is necessary because of the high reduction potential of tellurium that prevents the formation of telluride in the presence of zinc precursors, solvent, and ligand alone. The tellurium reduction was therefore performed using sodium borohydride and 1,4-butanediol under inert conditions. Then, glucose dissolved in 1,4-butanediol was added to quench excess borohydride. In the second step, contents of this flask were added to another flask containing ZnCl2, oleylamine, and octadecene at 100 °C. This gives ZnTe QDs, with the absorption spectra shown in Figure 1a. To grow CdS on ZnTe core, cadmium oleate and sulfur in oleylamine are used as cadmium and sulfur precursors, respectively. Dropwise addition of cadmium oleate and sulphur in oleylamine at 220 °C to a flask containing preformed ZnTe QDs under an inert atmosphere gives ZnTe/CdS. Absorption spectra of ZnTe/CdS QDs having different shell thicknesses are shown in Figure 1b. Because ZnTe/CdS is a type-II heterojunction core/shell QDs, there will be red shift in the band gap of the material as the CdS shell thickness increases. Figure 1c represents the XRD patterns of ZnTe/CdS having three different shell CdS thicknesses (shell thickness increases from bottom to top). The XRD pattern of the QDs having very thin CdS shell resembles the standard XRD pattern of ZnTe
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RESULTS AND DISCUSSION For single dot current versus voltage (I−V) spectra, an ambient atmosphere STM (Nano Rev STM 6.7, Sim 6) was used. The Pt/Ir tip of that STM was fixed on top of the nanocrystal film, and the spectra were collected under ambient atmospheric condition. The tip−sample distance was held constant throughout the I−V scan. For different STM images, the tunneling current parameter was set at 100 pA, while the bias was set at −3.0 V. Scan rate was set at 1400/s, while the gain was set at 2.0. 3177
DOI: 10.1021/acs.jpcc.7b12837 J. Phys. Chem. C 2018, 122, 3176−3181
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The Journal of Physical Chemistry C
Figure 2. STS data in normal and semilog scale with a device structure of (a) ITO/ZnO and (b) ITO/QDs; inset shows the schematic diagram for STM measurement with device structure. STM microgram of (c) ZnO and (d) ZnTe/CdS QDs with 7 ML.
The STS data of ITO/ZnO and ITO/QDs films are shown in Figure 2a,b, respectively. The STM micrographs of ZnO and ZnTe/CdS are in Figure 2c,d, respectively. The average grain size of ZnO is 15.6 nm, whereas average size of ZnTe/CdS QDs is 5.8 nm. In both cases of devices, I−V characteristics were taken at multiple points on the annealed surface of the ZnO (or QDs) layer. At one device terminal, charge carriers (either electrons or holes) tunnel between the ZnO (QDs) and the Pt/Ir tip of the STM, while at the other terminal, carriers move across the junction created between the layers of ZnO (QDs) and indium tin oxide (ITO). These STS plots show an asymmetry flow of current in the negative and positive bias regimes. This asymmetry is due to the different work functions of the device electrodes. Current of the positive substrate bias regime of the plot is due to the electron tunneling through the conduction band (CB) of ZnO (or QDs) to ITO, while in the negative substrate bias regime the current is due to the hole tunneling through the valence band (VB) of ZnO (or QDs) to ITO. As shown in Figure 2b, negligible current is observed before the turn-on voltage of the device. This is attributed to the absence of carrier tunneling in this regime. Prior to the attainment of turn on threshold, the tip Fermi level aligns with the band of the semiconductor. In the case of ITO/QD films, the “turn on” of tunneling current (both electron and hole) is a strong function of shell thickness. For hole tunneling, the magnitude of the turn on potential increases with shell thickness. Instead, for electrons, the magnitude of the turn-on potential decreases with increasing shell thickness. These two contrary trends are attributed to two different effects. In the
case of holes, the lowest energy state lies at the ZnTe valence band edge; however, this state is largely localized to the core. Holes thus tunnel into the QD and eventually relax into the core state. The CdS shell now acts as a barrier to the migration of holes from the QD core into the ITO layer. In the case of electrons, the opposing trend is observed because of quantum confinement of shell energy levels that raises the lowest-lying electronic state for the thinnest shells, thereby increasing the turn-on voltage. These data are shown schematically in Figure 3b,c. For hole tunneling this “turn on” is varied from −0.9 to −3.0 V, whereas for electron tunneling “turn on” varied from 1.9 to 3.3 V. Comparative STS data for ITO/n-ZnO/QDs film for various shell thickness QDs, like 2 to 7 ML shell, are shown in Figure 3a. All STS data have been taken on top of the individual QDs area because this area is the closest to the ideal case of isolated nanocrystals and was chosen for all measurements for rectification studies of individual colloidal QDs. During the I−V scan of the ITO/ZnO/QDs device, it was observed that there was negligible current (close to the noise level current) flow from the individual dot up to 10 V positive sample bias, indicating the lack of electron tunneling from the device. However, a sharp “turn on” in tunneling current was observed in the negative sample bias, which is indicative of the hole tunneling from the device. Overall, the STS studies conducted on a single dot show a clear rectification behavior. In addition to rectification, tunability of threshold voltage was achieved by controlling the shell thickness of the QDs. Threshold voltage increases with an increase in shell thickness. Comparative STS 3178
DOI: 10.1021/acs.jpcc.7b12837 J. Phys. Chem. C 2018, 122, 3176−3181
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Figure 3. (a) STS data with a device structure ITO/ZnO/QDs of various shell thickness of CdS shell on ZnTe core. Inset indicates the device structure for this measurement. Threshold voltage of this device gradually increases with shell thickness. Schematic diagram of type II core/shell heterostructure QDs and related band diagram with (b) thinner and (c) thicker CdS shell. This diagram also indicates the movement (and barrier) of hole from shell to core and core to shell for thinner and thicker shell QDs. (d−f) Schematic diagram of the model for the electronic structure and related current tunneling mechanism at the ZnO−QDs−tip interface at various substrate bias. Applied negative substrate bias changes band alignment, which permits the tunneling of holes through the VB of CdS shell to ZnTe core and ZnTe core to CdS shell, followed by tunneling through ZnO.
much better by using a model that is described in Figure 3d−f. In all of our experiments we have used ZnTe/CdS QDs with core size of ∼3.5 nm, which has a bandwidth ∼2.77 eV. On the contrary, work function of Pt/Ir is 5.2 eV. Before biasing the substrate, band alignment of ITO/ZnO/QDs/tip is shown in Figure 3d. In this diagram, EC and EV indicate the conduction and valence band edge energy levels of ZnO, whereas EF indicates the Fermi energy level of ZnO, which exists close to the EC. As soon as we apply negative bias to the substrate, band alignment changes to Figure 3e, which permits the tunneling of electron from CdS shell to the tip. Because of this electron tunneling a hole is generated in the CdS shell, which goes to the VB of core (ZnTe) of the QDs because of available lowest energy valence band in core (ZnTe). However, this hole also needs to be transferred to the VB of ZnO through the CdS shell barrier to get a tunneling current in the device. Because of the relative band alignment generated by external bias, the barrier of hole can be overcome after some threshold voltage, which results the high current in the STS data with a sharp turn on. However, this “turn on” can be tuned because of different shell (CdS) thickness that creates the barrier of hole-transfer CdS shell to the VB of ZnO. Negative sign of tunnelling current
data are shown in Figure 3a. From this Figure it is clearly observed that the threshold voltage of a single dot device with ITO/ZnO/QDs device structure gradually increases from 1.0 (for thin shell) to 3.0 V (for thick shell). Inset shows the device structure employed for this measurement. Tuning of the threshold voltage again occurs due to the variable barrier formation by CdS shell for hole tunneling by various shell thickness of QDs. As before, the hole relaxes into the core of the QD following injection. The transport of the hole from the QD to the ZnO layer, however, encounters a significant barrier from the CdS shell. This tunneling barrier is overcome by the external bias of the single dot device. In these circumstances, thicker shell acts as a faster transfer of hole from shell to the core and larger barrier width of tunneling of hole from core to shell part. Because the thicker shell creates a larger barrier width for hole tunneling to the shell part, which is followed by the hole transfer to the ZnO valence band (EV), higher turn-on voltage of the device is required. Thus by increasing shell thickness it is possible to increase threshold voltage gradually. This rectification behavior of the individual QDs of ITO/ ZnO/QDs film that is shown in Figure 3a can be understood 3179
DOI: 10.1021/acs.jpcc.7b12837 J. Phys. Chem. C 2018, 122, 3176−3181
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easily alter the tunneling current and threshold voltage of STS data. On the basis of this sensitive property, in the future it is possible to explore this device for single QD-based chemical or biosensor, which can show very fast response with high precision.
with negative substrate bias is indicative of the hole tunnelling after turn-on voltage. However, electron tunnelling, which is supposed to arise in positive substrate bias, is very negligible up to 10 V, indicating the lack of electron tunneling in the device. This is because of the relative position of tip work function (μtip) and the EC level of CdS, as shown in Figure 3f. With positive substrate bias, the position of CB of CdS also shifts in the upward direction along with tip energy level. Therefore, the tip energy level fails to align with the EC of CdS, resulting in the absence of electron tunnelling in the positive substrate bias regime. Such kind of negligibly low current in the positive sample bias regime is responsible for rectification behavior from single QD STS data. STS data in the semilogarithmic plot have been shown in Figure 4, which clearly demonstrates the ratio of tunneling
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12837. Details of synthesis of ZnTe/CdS core/shell QDs. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Bhola N. Pal: 0000-0001-7512-3441 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by IIT(BHU) internal funding under “Seed money grant” for new faculty. We are grateful to Physics Department, IIT(BHU) for providing the STM measurement facility. G.S.K. thanks IIT(BHU) for providing M. Tech. fellowship. We thank Mr. Satyaveer Singh for helping with different material characterization including XRD, AFM, TEM, and SEM. In addition, we thank “Central Instrument Facility Centre (CIFC)” of IIT(BHU) for providing the instrumental facility for AFM, TEM, and SEM studies.
Figure 4. Semilog plot of STS data with a device structure ITO/ZnO/ QDs for different CdS shell thickness devices. This plot shows the rectification ratio of ∼103 for all devices.
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CONCLUSIONS We have successfully fabricated an ambient atmosphere single colloidal quantum dot rectifying diode using a ZnTe/CdS type II core/shell QD with a rectification ratio of 103. Single dot device has been characterized with a device geometry ITO/ ZnO/QDs. Threshold voltage of the tunneling current has been tuned by changing the shell thickness of the QDs. This finding along with the rectification nature observed in these devices has been explained with one empirical model of relative band shifting of ZnO-QDs-tip interfaces with positive and negative substrate bias. In such kind of open atmospheric measurement, the tunneling current and threshold voltage strongly depends on the individual QDs. Any additional chemisorption or physisorption of molecule with QDs can 3180
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