This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Article Cite This: ACS Omega 2018, 3, 6224−6229
Quantum Dot Coupling in a Vertical Transport Device under Ambient Conditions Aviya Perlman Illouz,† Eyal Cohen,† Uri Peskin,‡ Shira Yochelis,† and Yossi Paltiel*,† †
Department of Applied Physics and the Center for Nanoscience and Nanotechnology, Hebrew University of Jerusalem, Jerusalem 9190401, Israel ‡ Schulich Faculty of Chemistry and Russell Berrie Nanotechnology Institute, Technion-Israel Institute of Technology, Haifa 3200003, Israel S Supporting Information *
ABSTRACT: The semiconductor device industry is constantly challenged by the demands of miniaturization. Therefore, the use of nanomaterials, such as quantum dots (QDs), is expected. At these scales, quantum effects are anticipated under industrial working conditions. Here, we present a simple fabrication method for integrating colloidal coupled QDs as components in a vertical device. Characterization of the fundamental properties of QDs as an ensemble of isolated particles and as layered QD hybrid structures is demonstrated. For the case of layered QD hybrid structures, coupling between dots is on average stronger with typical energy band gaps reduced by more than 200 meV. The shown device offers a straightforward method to measure and establish a strong coupling transport system under ambient conditions.
■
confinement energy12 and decreases the energy band gap.13 Realization of such a system was accomplished using QD superlattices,14 by encapsulating QDs in ordered matrices,15 or using molecular linkers.16−18 This effect can be simplified by considering a double quantum dot (DQD) model.19 In this model, charge carrier delocalization and energy-level splitting depend on the coupling that decays exponentially with the distance between neighboring QDs. Therefore, a DQD system presents a different DOS.20 In addition, the coupling between a DQD system and the conductive electrodes may reveal a unique negative differential resistance (NDR) effect due to destructive interference between the charge transport pathways through the electrodes.13 The use of self-assembled hybrid organic molecule-QD device21 opens a simple way to study the electronic properties of a QD ensemble under ambient conditions.22 In this work, we study the fundamental feature of coupling in layered QD hybrid structures linked by organic molecules in between the QDs, in a vertical device. The coupling properties are explored using multilayers of linker molecules and QDs organized by chemical adsorption cycles.
INTRODUCTION The semiconductor devices industry is constantly challenged to achieve better control on the nanoscale. Efforts to miniaturize electronic devices are directed in various directions, such as exploiting new materials and using self-assembly,1−4 which enables controlling the construction of solid structures on the atomic scale. At this scale, quantum phenomena begin to dominate, resulting in changed device behavior. For example, in closely adjoined semiconductor nanocrystals systems, quantum coupling must be taken into account.5 Quantum transport effects have also been shown to occur at room temperature in some cases.6,7 Colloidal quantum dots (QDs) are promising materials for realizing nanoscale semiconductor devices.8−10 QDs are semiconductor nanoparticles with three-dimensional quantum confinement of charge carriers due to their small size. As a result, the lower energy levels are discrete, similar to atoms, and therefore sometimes called “artificial atoms”. To integrate QDs in electronic devices, great importance lies in the ability to characterize their band structure, both as individuals and as aggregates. In previous works, scanning tunneling spectroscopy has been used to locally measure the single nanocrystal density of states (DOSs), usually at low temperatures.11 These results are acquired using a complicated measurement method and produce QD band structures far from the temperature of industrially operating devices. Moreover, the coupling between neighboring dots has not been considered. For densely assembled QD devices, evaluating the coupling between QDs is necessary. The coupling reduces the © 2018 American Chemical Society
■
EXPERIMENTAL METHODS Following Vortman et al.,22 a template device was realized using six layers: (1) a SiO2 substrate; (2) Au bottom contacts with an Received: May 23, 2018 Accepted: May 29, 2018 Published: June 11, 2018 6224
DOI: 10.1021/acsomega.8b00867 ACS Omega 2018, 3, 6224−6229
Article
ACS Omega
Figure 1. (a) Device scheme; the Au lower contacts are represented by the bottom golden squares. On the same sample, several devices are fabricated in parallel separated by an insulating layer (represented by the transparent color). The central rectangles show the micrometric holes in the insulating layer, enabling the molecules and QD adsorption directly on the Au contacts. The thin tunnel barrier layer and upper contacts are represented by the upper bands. (b) Cross section of the hybrid device layers of the separated micrometric holes. Voltage is applied between the bottom and top Au contacts, and the current that flows through the studied hybrid layers is measured.
Figure 2. Schematic of the proposed structures of the hybrid layers. (I) The QDs are connected to the Au substrate via self-assembled organic molecules. The QDs can then be uncoupled or coupled horizontally (with or without linker molecules) or vertically. Use of two adsorption cycles increases the density and allows for the creation of covalently coupled structures, such as those shown in (II) and (III).
Figure 3. Adsorption cycle characterization. HR-SEM images of the adsorbed 5 nm diameter CdSe core QDs: (a) after one adsorption cycle and (b) after the second adsorption cycle, where the QD density increases. (c) XPS measurements showing the differences in the counts of the Cd 3d states for the two adsorption cycles.
adhesion layer of 15 nm Cr; (3) self-assembled 1,9nonanedithiol organic molecules (purchased from SigmaAldrich); (4) 4.95 ± 0.25 nm diameter CdSe QDs (purchased from Sigma-Aldrich); (5) 6 nm Al2O3 as an insulating layer for tunneling; and (6) Au upper contacts. The contacts were fabricated using standard photolithography. The chemical adsorption cycles of CdSe QDs covalently bound by the linker molecules were performed using a self-assembly approach (further details in the Supporting Information). Figure 1a presents the scheme of the entire device. Figure 1b shows a layer cross section of the measured device.
Similar devices have been previously used to map the ensemble band structure of QDs using tunneling measurements.22 Here, we used the same device concept to compare different hybrid multilayers to explore the coupling properties in layered QD hybrid structures. Therefore, a first group of samples was immersed in one adsorption cycle of linker molecules and QDs, whereas a second group was immersed in two adsorption cycles. A reference sample for a closed packed assembly without linker molecules was prepared by simply dropping the QD solution on top of the Au contacts. These samples were realized using 3.75 ± 0.25 nm diameter CdSe QDs (purchased from MKnano). Electrical measurements were 6225
DOI: 10.1021/acsomega.8b00867 ACS Omega 2018, 3, 6224−6229
Article
ACS Omega
Figure 4. (a) I−V characteristics of the current through the studied layers. The current has been normalized to compare between different measurements. The range of current is nanoampere (one or two adsorption cycles of linker molecules and CdSe QDs, R ∼ 108 Ω). The zeroconductance region presents the QD band gap. Inset: Reference samples with only molecules present standard tunneling current, without the zerocurrent region. (b) Derivatives of the I−V curves. The curves are vertically offset for clarity. (c) Histograms of the measured energy band gaps. Two different energy groups are demonstrated around 2.2 and 1.7 eV. Error per device is estimated to be around 6% of the measured gap, about 0.1 eV.
cycle adsorption samples. The current has been normalized to compare between different measurements. The range of the current is nanoampere. For ease of interpretation, the derivatives of the I−V curves were taken, as presented in Figure 4b. In both cases, a clear NDR signal is measured. The zero-conductance region presents the QD band gap. All the measurements were taken with the same threshold of 10% of the 1.9 V current correlates to the optically measured band gap (see the discussion in paragraphs 12−15). The most significant error originates from the Fermi distribution widening at room temperature. The total error is estimated to be around 6% of the measured gap, about 0.1 eV, smaller than the measured differences between the samples. Peaks appearing at higher energies than the band gap might be related to the higher energy levels22 and are of no interest for the current discussion. Reference samples with only molecules display the standard tunneling current, without the zero-current region (inset of Figure 4a). In addition, The NDR effect does not appear in the reference measurements without the QDs. The NDR does appear in all the QDs samples, but changes from sample to sample. The I−V results show two typical graphs (blue and red in Figure 4a,b). For the sample with one adsorption cycle (blue), the graph displays a typical band gap of approximately 2.2 eV. The sample with two adsorption cycles (red) presents a smaller band gap of approximately 1.7 eV. This band gap is close to the CdSe bulk band gap of 1.74 eV at 300 K.25 For ease of comparison, histograms of the measured band gaps from various samples are shown in Figure 4c, evaluated from the zero-current regions. These histograms indicate two different energy groups. The first possesses band gaps of approximately
conducted on the samples by applying an electrical voltage between the Au contacts and measuring the current. We suggest that the consecutive adsorption of the selfassembled organic molecules and QDs changes the organization and distances between the QDs. Different organization will result in different coupling properties.23 The typical assemblies for one and two adsorption cycles are sketched in Figure 2: (I) transport through a single monolayer of QDs and (II and III) transport through covalently coupled QDs, horizontally or vertically, respectively. One adsorption cycle typically allows for the creation of structures presented by (I), with no coupling between the neighboring dots. In some occasions, we may have a different strength of horizontal coupling. Two adsorption cycles considerably increase the probability of a strong coupling between the neighboring QDs due to both increase in density (with no aggregation created)24 and second layer formation. The creation of two coupled dots in such structures is sketched in Figure 2II,III.
■
RESULTS AND DISCUSSION Figure 3 presents the high-resolution scanning electron microscopy (HR-SEM) and X-ray photoelectron spectroscopy (XPS) characterization of the QD density differences between the samples with one and two adsorption cycles. The QD density increases in the second cycle. The fabricated devices were characterized using standard current−voltage (I−V) measurements in a two-probe configuration. The voltage was applied between the top and bottom Au contacts. All the I−V measurements were taken at room temperature under ambient conditions. Figure 4a displays the representative I−V measurements for the one-cycle and two6226
DOI: 10.1021/acsomega.8b00867 ACS Omega 2018, 3, 6224−6229
Article
ACS Omega
Figure 5. (a) Absorption spectra of the layered CdSe QDs linked with organic molecules compared to the same isolated QDs in a solution. (Solution: abs. peak: 2.1 eV, half width at half maximum (HWHM): 0.06 eV; 10 layers: abs. peak: 2.08 eV, HWHM: 0.07 eV). The band gap peak of the layered QD hybrid structure is wider and possesses a broad low-energy tail extending to ∼1.7 eV. (b) Isolated CdSe QD (solution) PL (PL peak: 2.04 eV, full width at half-maximum (FWHM): 0.12 eV) compared to the absorption results (abs. peak: 2.07 eV, HWHM: 0.06 eV).
2.2 eV, whereas the second group displays smaller band gaps in the range of 1.7 eV. As shown in the histograms, the samples with one adsorption cycle exhibit energy band gaps from the two groups. The samples with two adsorption cycles present energy band gaps mainly from the “smaller band gap” group. Figure 5a compares the optical absorption spectra of colloidal, isolated CdSe QDs in the solution to the same linked QDs. The samples for the optical measurements were prepared using a layer-by-layer method26,27 to assemble the QDs covalently bound by 1,9-nonanedithiol linker molecules. In these samples, more than two layers were adsorbed to achieve sufficient signal,28 and only absorption measurements are presented, as the photoluminescence (PL) signal is too weak. We attribute the small PL to the coupling between QDs and the disorder that increases the nonradiative loses, which causes a shorter radiative lifetime.29 Small widening and a redshift of the band gap with a broad low energy tail extending to ∼1.7 eV are observed in the linked QDs, in agreement to previous studies.5,30 This tail can serve as an evidence for the strong coupling between the QDs in the sample, which causes the delocalization of excitons to the neighboring dots.31 It should be mentioned that light scattering from the samples is a possible contributor to the observed features. Previous work on this system showed that such a scattering mechanism is not the only cause for the low-energy tail.31 To relate the zero-conductance gap in the tunneling spectrum (also called the quasiparticle gap), Etun gap, to the optical band gap, Eopt gap, we must consider the electron−hole Coulomb attraction energy32
Accordingly, we can extract the band gap of the tunneling spectra and compare this value with the optical band gap of the isolated CdSe QDs in the solution. The band gap is 2.04 eV, as measured by PL in Figure 5b (green line). The absorption peak is shifted because of the Stokes shift. tun opt Egap = Egap + Je − h = 2.04 eV + 0.2 eV = 2.24 eV
This energy value matches the band gap of the first group, corresponding to the sample with one adsorption cycle, at approximately 2.2 eV. From this result, we can conclude that this band gap corresponds to the tunneling through isolated QDs. The second group of band gaps, at approximately 1.7 eV, corresponds to a dramatic reduction in the band gap. This observation may be attributed to pronounced interdot interactions present in the case of two adsorption cycles. Such interactions, associated with (at least partial) electronic delocalization over two (or more) QDs would indeed lead to a redshift in the optical band gap (as observed in Figure 5a, with a tail stretching up to 1.7 eV). Moreover, the Coulomb attraction correction (Je−h) should decrease in magnitude for delocalized electron and hole states. In transport I−V measurements through several channels, the higher current dominates the results. Therefore, the transport measurements of a distribution of band gaps will be mainly sensitive to the smaller energy band gap. For voltages above the lower channel gap, current will flow, and resistance will be nonzero. At higher voltages, additional channels will open, and the current will rise faster. In this case, unlike the absorption measurements, our device is mostly sensitive to the red tail of the spectra. We also believe that, due to the nonhomogeneous Al2O3 barrier, the current will mostly flow through the highest assembly of QDs, which also suggests that we mainly explored the highest-possible QD coupling in the device. According to this description, we suggest that, in the samples with only one adsorption cycle, the band gaps of an ensemble of isolated QDs are primarily measured, with a band gap matching the size distribution of the dots.22 As stated, the two adsorption cycles allow additional coupled structures (Figure 2). Thus, we attribute the smaller band gaps in the histogram of the samples with two adsorption cycles to tunneling through layered coupled QD. Note that in the samples with one adsorption cycle, a small band gap and NDR also appear in some cases. We attribute these features to the asymmetric horizontally coupled QD structures (Figure 2I). In all the structures, the top electrode is coupled to the system through
tun opt Egap − Je − h = Egap
Based on the electrostatic charging energy for a QD embedded in a homogeneous dielectric medium with an effective dielectric constant, ϵout,33,3433,34 the attraction energy can be estimated by35 ⎛ 1 0.79 ⎞ e 2 + Je − h = ⎜ ⎟ ϵ in ⎠ 4π ϵ0R ⎝ ϵout
For the CdSe QDs used in this study, the dielectric constant of the QDs, ϵin, is 8,36 and ϵout is treated as a free parameter. For QDs with a diameter of 5 nm with ϵout = 2.1, this theoretical calculation predicts a value of Je−h = 0.3 eV, which is in good agreement with the observed values of Je−h (D = 5 nm) ∼ 0.2 eV (combining scanning tunneling spectroscopy and optical measurements).36 6227
DOI: 10.1021/acsomega.8b00867 ACS Omega 2018, 3, 6224−6229
Article
ACS Omega
Figure 6. (a) Isolated 3.75 ± 0.25 nm CdSe QD (solution) photoluminescence results (PL peak: 2.12 eV, FWHM: 0.1 eV) and absorption results (abs. peak: 2.16 eV, HWHM: 0.06 eV). (b) I−V characteristics and derivative (inset) of the current that flows through the studied CdSe QD layer using adsorption by dropping the QDs on the sample without ligand exchange (R ∼ 1011 Ω). (c) Histogram of the measured energy band gaps, demonstrating a wide distribution.
■
CONCLUSIONS We presented a simple fabrication method for integrating colloidal coupled QDs as components in a vertical device. Characterization of the fundamental properties of the QDs as an ensemble of isolated particles and as layered QD hybrid structures was demonstrated. The double dipped samples presented typical smaller energy band gaps by more than 200 meV compared to the samples with one adsorption cycle. These results indicate the strong coupling between dots in most double dipped samples. The device presents a straightforward method to establish and measure the strong coupling in a layered QD hybrid system.
an Al2O3 tunnel barrier, whereas the bottom electrode is more strongly coupled to the dots via the organic linkers. This asymmetry was predicted to cause a NDR effect.13 The QD distribution ensures different dot sizes upon the coupling of two adjacent QDs. Both the NDR and the band gap in the simple studied device are clearly sensitive to changes in the coupling of the assembled structure. Finally, to compare the results of the adsorbed layers to the isolated QD system, we prepared the samples by dropping the 3.75 ± 0.25 nm diameter CdSe QD solution on the sample with only the spacer ligands of the QDs and no linker molecules. The typical ligands of the QDs (octadecyl amine, for the MKnano particles; trioctylphosphine oxide, for the SigmaAldrich particles) are long and introduce large distances between the QDs.37 Without the ligand exchange, no coupled structures are expected due to the spatial separation and lack of covalent bonds. Indeed, the I−V characteristic of those measurements (presented in Figure 6b) exhibit large zerocurrent gaps of 2.4 eV. These values correspond to isolated CdSe QDs in a solution, as optically measured and presented in Figure 6a (green line) via the Coulomb attraction Je−h (D = 4 nm) ∼ 0.3 eV.36 The measured resistance of these samples was relatively large, of the order of R ∼ 1011 Ω, due the higher insulation of the samples. The histogram presents (Figure 6c) a wide distribution of gaps with respect to the different possible current channels. From an estimation of the coupling energy due to the band gap changes, we achieve energies larger than 200 meV, i.e., confirming the presence of a strong coupling regime. This energy value is greater than the optically measured energy of approximately ∼100 meV.5 We attribute the strong coupling to the ligand exchange, which enables the formation of covalent bonds. In addition, as previously stated, we mainly explored the highest-possible QD coupling in the device. In a tight package configuration with our QDs and ligands, taking the nearest neighbors’ delocalization of the wavefunction (15 nm delocalization) is enough to create such a shift in the energy. Therefore, the appropriate comparison relates to the lowenergy absorption tail more than to the redshifted energy peak. Moreover, this value is closer to the larger reductions in the band gaps of QDs in the arrays. In some cases, the energy decreases of up to ∼400 meV12,38 were detected in the tunneling spectra. The NDR also indicates the presence of the strong coupling regime.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00867. Photolithography sample preparation; self-assembly of the organic molecules and QDs (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank the COPAC FET Open #766563 for funding the research.
■
REFERENCES
(1) Burrows, P. E.; Forrest, S. R.; Thompson, M. E. Prospects and Applications for Organic Light-Emitting Devices. Curr. Opin. Solid State Mater. Sci. 1997, 2, 236−243. (2) Naaman, R. Molecular Controlled Nano-Devices. Phys. Chem. Chem. Phys. 2011, 13, 13153−13161. (3) Katz, H. E.; Bao, Z. The Physical Chemistry of Organic FieldEffect Transistors. J. Phys. Chem. B 2000, 104, 671−678. (4) Tao, N. J. Electron Transport in Molecular Junctions. Nat. Nanotechnol. 2006, 1, 173−181. (5) Crisp, R. W.; Schrauben, J. N.; Beard, M. C.; Luther, J. M.; Johnson, J. C. Coherent Exciton Delocalization in Strongly Coupled Quantum Dot Arrays. Nano Lett. 2013, 13, 4862−4869.
6228
DOI: 10.1021/acsomega.8b00867 ACS Omega 2018, 3, 6224−6229
Article
ACS Omega (6) Collini, E.; Scholes, G. D. Electronic and Vibrational Coherences in Resonance Energy Transfer along MEH-PPV Chains at Room Temperature. J. Phys. Chem. A 2009, 113, 4223−4241. (7) Collini, E.; Scholes, G. D. Coherent Intrachain Energy Migration in a Conjugated Polymer at Room Temperature. Science 2009, 323, 369−373. (8) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Shape Control of CdSe Nanocrystals. Nature 2000, 404, 59−61. (9) Klein, D. L.; Roth, R.; Lim, A. K.; Alivisatos, A. P.; McEuen, P. L. A Single-Electron Transistor Made from a Cadmium Selenide Nanocrystal. Nature 1997, 389, 699−701. (10) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Light-Emitting Diodes Made from Cadmium Selenide Nanocrystals and a Semiconducting Polymer. Nature 1994, 354−357. (11) Banin, U.; Cao, Y.; Katz, D.; Millo, O. Identification of Atomiclike Electronic States in Indium Arsenide Nanocrystal Quantum Dots. Nature 1999, 400, 542−544. (12) Liljeroth, P.; Overgaag, K.; Urbieta, A.; Grandidier, B.; Hickey, S. G.; Vanmaekelbergh, D. Variable Orbital Coupling in a TwoDimensional Quantum-Dot Solid Probed on a Local Scale. Phys. Rev. Lett. 2006, 97, No. 096803. (13) Pozner, R.; Lifshitz, E.; Peskin, U. Negative Differential Resistance Probe for Interdot Interactions in a Double Quantum Dot Array. J. Phys. Chem. Lett. 2015, 6, 1521−1528. (14) Kagan, C. R.; Murray, C. B. Charge Transport in Strongly Coupled Quantum Dot Solids. Nat. Nanotechnol. 2015, 1013−1026. (15) Kinder, E.; Moroz, P.; Diederich, G.; Johnson, A.; Kirsanova, M.; Nemchinov, A.; O’Connor, T.; Roth, D.; Zamkov, M. Fabrication of All-Inorganic Nanocrystal Solids through Matrix Encapsulation of Nanocrystal Arrays. J. Am. Chem. Soc. 2011, 133, 20488−20499. (16) Zhang, J.; Tolentino, J.; Smith, E. R.; Zhang, J.; Beard, M. C.; Nozik, A. J.; Law, M.; Johnson, J. C. Carrier Transport in PbS and PbSe QD Films Measured by Photoluminescence Quenching. J. Phys. Chem. C 2014, 118, 16228−16235. (17) Lee, J.-S.; Kovalenko, M. V.; Huang, J.; Chung, D. S.; Talapin, D. V. Band-like Transport, High Electron Mobility and High Photoconductivity in All-Inorganic Nanocrystal Arrays. Nat. Nanotechnol. 2011, 6, 348−352. (18) Cohen, E.; Gruber, M.; Romero, E.; Yochelis, S.; van Grondelle, R.; Paltiel, Y. Properties of Self-Assembled Hybrid Organic Molecule/ Quantum Dot Multilayered Structures. J. Phys. Chem. C 2014, 118, 25725−25730. (19) Pozner, R.; Lifshitz, E.; Peskin, U. Charge Transport-Induced Recoil and Dissociation in Double Quantum Dots. Nano Lett. 2014, 14, 6244−6249. (20) Schedelbeck, G.; Wegscheider, W.; Bichler, M.; Abstreiter, G. Coupled Quantum Dots Fabricated by Cleaved Edge Overgrowth: From Artificial Atoms to Molecules. Science 1997, 278, 1792−1795. (21) Aqua, T.; Naaman, R.; Aharoni, A.; Banin, U.; Paltiel, Y. Hybrid Nanocrystals-Organic-Semiconductor Light Sensor. Appl. Phys. Lett. 2008, 92, No. 223112. (22) Vortman, S.; Ben-dor, O.; Yochelis, S.; Amit, Y.; Paltiel, Y. Mapping the Energy Band Structure of Nanocrystal Monolayers under Ambient Conditions. J. Phys. Chem. C 2013, 117, 22245−22249. (23) Gotesman, G.; Naaman, R. Temperature-Dependent Coupling in Hybrid Structures of Nanoparticle Layers Linked by Organic Molecules. J. Phys. Chem. Lett. 2010, 1, 594−598. (24) Neubauer, A.; Yochelis, S.; Popov, I.; Ben Hur, A.; Gradkowski, K.; Banin, U.; Paltiel, Y. Local Cathode Luminescence Resonant Peak in Hybrid Organic Nanocrystal Systems. J. Phys. Chem. C 2012, 116, 15641−15645. (25) Kittel, C. Introduction to Solid State Physics, 8th ed.; Wiley: Hoboken, NJ, 2004. (26) Kotov, N. A.; Dekany, I.; Fendler, J. H. Layer-by-Layer SelfAssembly of Polyelectrolyte-Semiconductor Nanoparticle Composite Films. J. Phys. Chem. 1995, 99, 13065−13069.
(27) Ouyang, M.; Awschalom, D. D. Coherent Spin Transfer between Molecularly Bridged Quantum Dots. Science 2003, 301, 1074−1078. (28) Galanty, M.; Yochelis, S.; Stern, L.; Dujovne, I.; Levy, U.; Paltiel, Y. Extinction Enhancement from a Self-Assembled Quantum Dots Monolayer Using a Simple Thin Films Process. J. Phys. Chem. C 2015, 119, 24991−24995. (29) Cohen, E.; Komm, P.; Rosenthal-Strauss, N.; Dehnel, J.; Lifshitz, E.; Yochelis, S.; Levine, R. D.; Remacle, F.; Fresch, B.; Marcus, G.; Paltiel, Y. Fast Energy Transfer in CdSe Quantum Dot Layered Structures: Controlling Coupling with Covalent-Bond Organic Linkers. J. Phys. Chem. C 2018, 122, 5753−5758. (30) Luther, J. M.; Law, M.; Song, Q.; Perkins, C. L.; Beard, M. C.; Nozik, A. J. Structural, Optical, and Electrical Properties of SelfAssembled Films of PbSe Nanocrystals Treated with 1,2-Ethanedithiol. ACS Nano 2008, 2, 271−280. (31) Cohen, E.; Gdor, I.; Romero, E.; Yochelis, S.; van Grondelle, R.; Paltiel, Y. Achieving Exciton Delocalization in Quantum Dot Aggregates Using Organic Linker Molecules. J. Phys. Chem. Lett. 2017, 8, 1014−1018. (32) Franceschetti, A.; Zunger, A. Pseudopotential Calculations of Electron and Hole Addition Spectra of InAs, InP, and Si Quantum Dots. Phys. Rev. B 2000, 62, 2614−2623. (33) Banin, U.; Millo, O. Tunneling and Optical Spectroscopy of Semiconductor Nanocrystals. Annu. Rev. Phys. Chem. 2003, 54, 465− 492. (34) Franceschetti, A.; Zunger, A. Addition Energies and Quasiparticle Gap of CdSe Nanocrystals. Appl. Phys. Lett. 2000, 76, 1731−1733. (35) Niquet, Y. M.; Delerue, C.; Allan, G.; Lannoo, M. Interpretation and Theory of Tunneling Experiments on Single Nanostructures. Phys. Rev. B 2002, 65, No. 165334. (36) Jdira, L.; Liljeroth, P.; Stoffels, E.; Vanmaekelbergh, D.; Speller, S. Size-Dependent Single-Particle Energy Levels and Interparticle Coulomb Interactions in CdSe Quantum Dots Measured by Scanning Tunneling Spectroscopy. Phys. Rev. B 2006, 73, No. 115305. (37) Kovalenko, M. V.; Scheele, M.; Talapin, D. V. Colloidal Nanocrystals with Molecular Metal Chalcogenide Surface Ligands. Science 2009, 324, 1417−1420. (38) Steiner, D.; Aharoni, A.; Banin, U.; Millo, O. Level Structure of InAs Quantum Dots in Two-Dimensional Assemblies. Nano Lett. 2006, 6, 2201−2205.
6229
DOI: 10.1021/acsomega.8b00867 ACS Omega 2018, 3, 6224−6229
