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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 25 — Dec. 10, 2007
  • pp: 17163–17170
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Near-field photodetection with high spatial resolution by nanocrystal quantum dots

M. Hegg and L. Y. Lin  »View Author Affiliations


Optics Express, Vol. 15, Issue 25, pp. 17163-17170 (2007)
http://dx.doi.org/10.1364/OE.15.017163


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Abstract

We report a new demonstration of nanoscale solution-processed photodetectors by fabricating a nano-sized gap between two electrodes and drop-casting nanocrystal quantum dots (NCQDs) into the gap. We demonstrate a detection sensitivity of 62 pW with a max responsivity of 2.7 mA/W over a device with a nano-gap of 25 nm. Additionally, we characterize the dependence of signal-to-dark current ratio and responsivity on nano-gap size. Responsivity ranges from 1–90 mA/W for a nano-gap size range of 25 nm–1.5 nm. Our results represent the first demonstration of how near-field optical detection for sub-diffraction nanophotonic integrated circuits can be achieved in principle using NCQDs.

© 2007 Optical Society of America

1. Introduction

Fig. 1. Energy diagram of NCQD a, in the absence of light b, in the presence of light. ħω0 is the photon energy which promotes an electron from the valence to conduction band, Γ is the tunneling rate for a carrier in the conduction band of the NCQD, and Γ12. c, Schematic I–V characteristics of a nano-scale NCQD photodetector.

2. Fabrication

Figure 2 shows a schematic of the fabrication steps involved in constructing a nano-scale quantum dot photodetector. A Si wafer with a 1 µm surface layer of SiO2 is used as the substrate. The wafer is spin-coated with a 100 nm layer of polymethyl methacrylate (PMMA) photoresist and patterned by electron-beam lithography (EBL) to make narrow continuous electrode patterns with line widths of 50 nm connected to large-area contact pads of 50×50 µm. After developing the PMMA, the exposed SiO2 is silanized with a monolayer of (3-Mercaptopropyl)trimethoxysilane (MPTMS). To perform silanization, the wafer is oxygenplasma cleaned and then exposed to the MPTMS gaseous molecules inside a vacuum dessicator for two hours. The sample is then immediately transferred to a thermal evaporator for deposition of a 300-Å Au layer, followed by lift-off of PMMA to form the Au electrodes. The electrodes are broken to form a nano-gap by continuously increasing a voltage across the electrodes at a rate of 0.1 mV/sec. After the onset of electromigration [29

29. A. K. Mahapatro, S. Ghosh, and D. B. Janes, “Nanometer scale electrode separation (nanogap) using electromigration at room temperature,” IEEE Transactions on Nanotechnology 5, 232–236 (2006). [CrossRef]

], the conductance is monitored until a deviation by 90% from the original conductance is observed, at which point the applied bias is turned off. After confirmation of the break using scanning electron microscopy (SEM), a 100 µM solution of CdSe/ZnS NCQDs in toluene is drop-cast onto the wafer. The NCQDs have a nominal diameter (core+shell) of 5.2 nm, an emission wavelength of 620 nm, and are capped with trioctylphosphine oxide (TOPO) to passivate surface trap states and reduce nanocrystal aggregation.

The SiO2 layer insulates the device from photo-generated carriers in the Si. The MPTMS monolayer acts as an insulating adhesion layer for the Au film [30

30. A. K. Mahapatro, A. Scott, A. Manning, and D. B. Janes, “Gold surface with sub-nm roughness realized by evaporation on a molecular adhesion monolayer,” Appl. Phys. Lett. 88, 151917, (2006). [CrossRef]

]. I–V measurements show that MPTMS works better as the adhesion layer than regular metal such as Cr or Ti during the break-junction process, as regular metal often leaves a conduction trace and results in high dark current (~10’s of nA compared to less than 1 pA using MPTMS). We use a drop-casting method to deposit the QDs between electrodes. This method results in a thin film of QDs over a large area, but only QDs in and around the nano-gap can contribute significantly to conduction (to be discussed in Sec. 4). A more location-specific deposition of the QDs can be achieved using thiol molecule self-assembly, which would bind the QDs to Au only [31

31. D. L. Klein, R. Roth, A. K. L. Lim, A. P. Alivisatos, and P. L. Mceuen, “A single-electron transistor made from a cadmium selenide nanocrystal,” Nature 389, 699–701 (1997). [CrossRef]

], although such a process will result in large distances between the QDs and low tunneling currents when the gap between the electrodes is larger.

Fig. 2. Fabrication process of the NCQD photodetector. a, EBL and Au deposition with MPTMS as the adhesion layer. b, creating a nano-gap by break-junction procedure, and c, drop-casting QD deposition.

Electrodes having widths of 50 nm were broken with gap sizes ranging from 1.5–25 nm, in order to demonstrate the flexibility of the break-junction procedure. Figure 3(a) shows a large-area device structure and the magnified nano-gap region, revealing a narrow break-junction with 25 nm gap size. The large 100×150 µm contact pads are for probe-testing purpose only and are not an essential part of the active device. The break-junction process was chosen to achieve proof-of-concept demonstration of the device, but it requires breaking the electrodes in series. In principle, high-resolution EBL can also be used to pattern the nano-gap electrodes and eliminate the break-junction step [32

32. L. Huang, M. C. Hegg, C.-J. Wang, and L. Y. Lin, “Fabrication of a nanophotonic waveguide and photodetector integrated device,” Micro and Nano Letters (to be published)

]. Figure 3(b) shows an example of a device after drop-casting NCQDs into the nano-gap. The NCQDs fill the gap and the surrounding area of the device. Although QDs are deposited in a large area, only tunneling in and around the narrow break-junction gap can contribute significantly to the measured current.

Fig. 3. Fabrication results. a, Scanning electron micrograph (SEM) of a pre-QD deposition break-junction electrode. b, SEM of a NCQD photodetector.

3. Results

Figure 4(a) shows control measurements performed on the 25 nm gap device shown in Fig. 3(a) prior to quantum dot deposition. I–V measurements with and without optical excitation were performed in order to demonstrate the nano-gap insensitivity to incident radiation. The results show that the measured tunneling current, either under illumination or dark condition, is less than 1 pA without the QDs throughout a bias range of 0–5V. The same measurement is then repeated for the device after NCQD deposition. Figure 4(b) shows the I–V characteristics of the device under illumination and dark conditions. The illumination intensity is 1 pW/nm2. The tunneling current under dark conditions increases after the QDs are attached but still remains below 4 pA, confirming the placement of QDs between the electrodes. The photocurrent at 4 V is over 3 times larger than the dark current, confirming photosensitivity of the device. The specific contact resistance is the reciprocal of the slope of the dark current I–V curve at zero bias multiplied by the device cross-sectional area, and is calculated to be Rc=16 Ωcm2. The linearity of the dark current I–V curve and the low value for Rc indicate good ohmic contacts.

Fig. 4. Characterization of NCQD photodetectors. a, I–V characteristics of the break-junction electrode in dark (squares) and illuminated (circles) conditions. b, I–V characteristics of the NCQD photodetector under dark (squares) and illuminated (circles) conditions.

To properly calculate a device responsivity, an effective device area was estimated by simulating the electric field intensity in and around the nanogap. Fig. 5(a) shows a surface plot of the E-field intensity in and around a 20 nm nano-gap. The E-field is highest around the edges of the gap and decreases radially outward from the center. Fig. 5(b) shows how the E-field intensity decays rapidly with radial distance from the gap edges. For a 20 nm gap, E/Emax is approximately 10% when r=50 nm. The photocurrent is a function of electric field, number of QDs, and number of tunneling barriers between QDs. The electrode-to-electrode distance increases abruptly from <30 nm to >10 µm outside the nano-gap region. Although the total number of QDs increases, the distance between electrodes and the number of tunneling barriers both increase proportionally, which contribute negatively to the photocurrent. We therefore define the effective area of the device as the rectangular area of the nano-gap plus a circular area w/radius r defined by the distance from the electrode edge at which the E-field is 10% of the highest value. This distance scales linearly with gap size.

Fig. 5. Finite element simulation of the electric field in and around a 20nm nano-gap. a, Surface plot of the electric field intensity in and around the nano-gap. b, Plot of E/Emax vs. radial distance from the electrode edge.

To characterize the sensitivity and responsivity of the device, photocurrent measurements were performed under various illumination intensities, with the device biased at 4 V. Optical signal used for excitation is from a 405 nm laser source coupled to the device through a lensed fiber probe and focused to a spot size diameter of 100 µm. Current is measured using a Keithley 6430 sub-femtoamp source meter and the device is electrically shielded on a low-noise Cascade Microtech probe station. The results are shown in figure 6(a). The device starts to show measurable photocurrent of 80 fA under 62 pW illumination over the effective device area. No special shielding of ambient light was done for this measurement. The device has an average responsivity of 2.7 mA/W under low-intensity illumination, and starts to exhibit saturation effects when the optical power increases beyond 4 nW. The noise effective power (NEP) is the noise current divided by the responsivity under the same experimental conditions. From prior research of large devices with 5 um electrode spacing [24

24. G. Konstantatos, I. Howard, A. Fischer, S. Hoogland, J. Clifford, E. Klem, L. Levina, and E. H. Sargent, “Ultrasensitive solution-cast quantum dot photodetectors,” Nature 442, 180–183 (2006). [CrossRef] [PubMed]

], the noise floor for 10 nA of dark current is approximately 0.1 pA/Hz1/2. The device shown in Figure 4 exhibits 3.67 pA of dark current at 4V bias with a corresponding responsivity of 2.7 mA/W. If a noise floor of 0.1 pA/Hz1/2 is used, this equates to an NEP of 3.7×10-11 W/Hz1/2. This is effectively a lower bound for the NEP of our device. As an upper bound, if our device is shot noise-limited, the noise floor for 3.67 pA is (2qIdB)1/2=1.08 fA/Hz1/2 which equates to an NEP of 4.01×10-13 W/Hz1/2.

Several devices with a range of gap sizes from 1.5–25 nm were fabricated and tested. Figure 6(b) shows the signal-to-dark current ratio (SDR) and responsivity (R) of these devices as a function of gap size. The largest measured SDR is 3.5 for a gap of 25 nm and decreases with smaller gap size due to an increase in the dark current as the number of tunneling barriers between the electrodes is reduced. The largest device responsivity is nearly 0.1 A/W over the effective device area for a gap size of 1.5 nm and decreases with increasing gap size. This is because the absorbed optical power increases linearly with the gap size, but the tunneling photocurrent does not.

Fig. 6. Characterization results of the NCQD photodetector. a, Responsivity measured at room temperature using a calibrated 405 nm laser and biased at 4.0 V. b, Signal-to-dark current ratio SDR (squares) and responsivity R (circles) at 4V bias for devices of variable gap sizes.

Integrated fabrication of the QD waveguides and the QD photodetector has been demonstrated [32

32. L. Huang, M. C. Hegg, C.-J. Wang, and L. Y. Lin, “Fabrication of a nanophotonic waveguide and photodetector integrated device,” Micro and Nano Letters (to be published)

]. Figure 7 shows the SEM of the integrated device as the first step towards high-density photonic integrated circuits.

Fig. 7. Integrated photodetector and waveguide device.

4. Conclusion

Acknowledgments

We thank L. Huang for the SEM images shown in figure 3(b) and figure 7. M.C.H. thanks the NSF IGERT Graduate Fellowship Program and University of Washington UIF Graduate Fellowship Program for financial support. Work was performed in part at the University of Washington Nanotech User Facility (NTUF), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation.

References and links

1.

S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, “Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides,” Nature Materials 2, 229–232 (2003). [CrossRef] [PubMed]

2.

S. A. Maier, P. E. Barclay, T. J. Johnson, M. D. Friedman, and O. Painter, “Low-loss fiber accessible plasmon waveguide for planar energy guiding and sensing,” Appl. Phys. Lett. 84, 3990–3992 (2004). [CrossRef]

3.

T. Yatsui, W. Nomura, and M. Ohtsu, “Self-assembly of size- and position-controlled ultralong nanodot chains using near-field optical desorption,” Nano Lett. 5, 2548–2551 (2005). [CrossRef] [PubMed]

4.

C. J. Barrelet, A. B. Greytak, and C. M. Lieber, “Nanowire photonic circuit elements,” Nano Lett. 4, 1981–1985 (2004). [CrossRef]

5.

E. Yablonovitch, “Photonic crystals - Towards rational material design,” Nature Materials 2, 648–649 (2003). [CrossRef] [PubMed]

6.

M. F. Yanik, S. H. Fan, M. Soljacic, and J. D. Joannopoulos, “All-optical transistor action with bistable switching in a photonic crystal cross-waveguide geometry,” Opt. Lett. 28, 2506–2508 (2003). [CrossRef] [PubMed]

7.

C. A. Barrios, V. R. Almeida, R. Panepucci, and M. Lipson, “Electrooptic modulation of silicon-on-insulator submicrometer-size waveguide devices,” IEEE J. Lightwave Technol. 21, 2332–2339 (2003). [CrossRef]

8.

M. Law, D. J. Sirbuly, J. C. Johnson, J. Goldberger, R. J. Saykally, and P. D. Yang, “Nanoribbon waveguides for subwavelength photonics integration,” Science 305, 1269–1273 (2004). [CrossRef] [PubMed]

9.

M. Ohtsu, K. Kobayashi, T. Kawazoe, S. Sangu, and T. Yatsui, “Nanophotonics: Design, fabrication, and operation of nanometric devices using optical near fields,” IEEE J. Sel. Top. Quantum Electron. 8, 839–862 (2002). [CrossRef]

10.

J. F. Wang, M. S. Gudiksen, X. F. Duan, Y. Cui, and C. M. Lieber, “Highly polarized photoluminescence and photodetection from single indium phosphide nanowires,” Science 293, 1455–1457 (2001). [CrossRef] [PubMed]

11.

O. Hayden, R. Agarwal, and C. M. Lieber, “Nanoscale avalanche photodiodes for highly sensitive and spatially resolved photon detection,” Nature Materials 5, 352–356 (2006). [CrossRef] [PubMed]

12.

M. Freitag, Y. Martin, J. A. Misewich, R. Martel, and P. H. Avouris, “Photoconductivity of single carbon nanotubes,” Nano Lett. 3, 1067–1071 (2003). [CrossRef]

13.

O. Astafiev, S. Komiyama, T. Kutsuwa, V. Antonov, Y. Kawaguchi, and K. Hirakawa, “Single-photon detector in the microwave range,” Appl. Phys. Lett. 80, 4250–4252 (2002). [CrossRef]

14.

J. Alda, J. M. Rico-Garcia, J. M. Lopez-Alonso, and G. Boreman, “Optical antennas for nano-photonic applications,” Nanotechnology 16, S230–S234 (2005). [CrossRef]

15.

K. T. Posani, V. Tripathi, S. Annamalai, N. R. Weisse-Bernstein, S. Krishna, R. Perahia, O. Crisafulli, and O. J. Painter, “Nanoscale quantum dot infrared sensors with photonic crystal cavity,” Appl. Phys. Lett. 88, 151104, (2006). [CrossRef]

16.

A. P. Alivisatos, “Semiconductor clusters, nanocrystals, and quantum dots,” Science 271, 933–937 (1996). [CrossRef]

17.

C. B. Murray, D. J. Norris, and M. G. Bawendi, “Synthesis and Characterization of Nearly Monodisperse Cde (e=S, Se, Te) Semiconductor Nanocrystallites,” J. Am. Chem. Soc. 115, 8706–8715 (1993). [CrossRef]

18.

G. M. Whitesides, J. P. Mathias, and C. T. Seto, “Molecular Self-Assembly and Nanochemistry - A Chemical Strategy for the Synthesis of Nanostructures,” Science 254, 1312–1319 (1991). [CrossRef] [PubMed]

19.

R. D. Schaller, M. Sykora, S. Jeong, and V. I. Klimov, “High-efficiency carrier multiplication and ultrafast charge separation in semiconductor nanocrystals studied via time-resolved photoluminescence,” J. Phys. Chem. B 110, 25332–25338 (2006). [CrossRef] [PubMed]

20.

R. D. Schaller, M. Sykora, J. M. Pietryga, and V. I. Klimov, “Seven excitons at a cost of one: Redefining the limits for conversion efficiency of photons into charge carriers,” Nano Lett. 6, 424–429 (2006). [CrossRef] [PubMed]

21.

E. J. Gansen, M. A. Rowe, M. B. Greene, D. Rosenberg, T. E. Harvey, M. Y. Su, R. H. Hadfield, S. W. Ham, and R. P. Mirin, “Photon-number-discriminating detection using a quantum-dot, optically gated, field-effect transistor,” Nature Photonics 1, 585–588 (2007). [CrossRef]

22.

M. Drndic, M. V. Jarosz, N. Y. Morgan, M. A. Kastner, and M. G. Bawendi, “Transport properties of annealed CdSe colloidal nanocrystal solids,” J. Appl. Phys. 92, 7498–7503 (2002). [CrossRef]

23.

N. Y. Morgan, C. A. Leatherdale, M. Drndic, M. V. Jarosz, M. A. Kastner, and M. Bawendi, “Electronic transport in films of colloidal CdSe nanocrystals,” Phys. Rev. B 66, 075339, (2002). [CrossRef]

24.

G. Konstantatos, I. Howard, A. Fischer, S. Hoogland, J. Clifford, E. Klem, L. Levina, and E. H. Sargent, “Ultrasensitive solution-cast quantum dot photodetectors,” Nature 442, 180–183 (2006). [CrossRef] [PubMed]

25.

G. Konstantatos, J. Clifford, L. Levina, and E. H. Sargent, “Sensitive solution-processed visible-wavelength photodetectors,” Nature Photonics 1, 531–534 (2007). [CrossRef]

26.

C. J. Wang, L. Huang, B. A. Parviz, and L. Y. Lin, “Subdiffraction photon guidance by quantum-dot cascades,” Nano Lett. 6, 2549–2553 (2006). [CrossRef] [PubMed]

27.

David Ginger and Neil Greenham, “Electrical Properties of Semiconductor Nanocrystals,” in Semiconductor and Metal Nanocrystals: Synthesis and Electronic and Optical Properties, Victor I. Klimov, ed., (Marcel Dekker, Inc., New York, 2004), pp. 239–288.

28.

H. C. Liu and G. C. Aers, “Resonant Tunneling Through One-Dimensional, Two-Dimensional, and 3-Dimensionally Confined Quantum Wells,” J. Appl. Phys. 65, 4908–4914 (1989). [CrossRef]

29.

A. K. Mahapatro, S. Ghosh, and D. B. Janes, “Nanometer scale electrode separation (nanogap) using electromigration at room temperature,” IEEE Transactions on Nanotechnology 5, 232–236 (2006). [CrossRef]

30.

A. K. Mahapatro, A. Scott, A. Manning, and D. B. Janes, “Gold surface with sub-nm roughness realized by evaporation on a molecular adhesion monolayer,” Appl. Phys. Lett. 88, 151917, (2006). [CrossRef]

31.

D. L. Klein, R. Roth, A. K. L. Lim, A. P. Alivisatos, and P. L. Mceuen, “A single-electron transistor made from a cadmium selenide nanocrystal,” Nature 389, 699–701 (1997). [CrossRef]

32.

L. Huang, M. C. Hegg, C.-J. Wang, and L. Y. Lin, “Fabrication of a nanophotonic waveguide and photodetector integrated device,” Micro and Nano Letters (to be published)

OCIS Codes
(040.0040) Detectors : Detectors
(040.5160) Detectors : Photodetectors
(230.5590) Optical devices : Quantum-well, -wire and -dot devices
(270.5570) Quantum optics : Quantum detectors

ToC Category:
Detectors

History
Original Manuscript: November 5, 2007
Revised Manuscript: December 3, 2007
Manuscript Accepted: December 4, 2007
Published: December 7, 2007

Citation
M. Hegg and L. Y. Lin, "Near-field photodetection with high spatial resolution by nanocrystal quantum dots," Opt. Express 15, 17163-17170 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-25-17163


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References

  1. S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, "Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides," Nat. Mater. 2, 229-232 (2003). [CrossRef] [PubMed]
  2. S. A. Maier, P. E. Barclay, T. J. Johnson, M. D. Friedman, and O. Painter, "Low-loss fiber accessible plasmon waveguide for planar energy guiding and sensing," Appl. Phys. Lett. 84, 3990-3992 (2004). [CrossRef]
  3. T. Yatsui, W. Nomura, and M. Ohtsu, "Self-assembly of size- and position-controlled ultralong nanodot chains using near-field optical desorption," Nano Lett. 5, 2548-2551 (2005). [CrossRef] [PubMed]
  4. C. J. Barrelet, A. B. Greytak, and C. M. Lieber, "Nanowire photonic circuit elements," Nano Lett. 4, 1981- 1985 (2004). [CrossRef]
  5. E. Yablonovitch, "Photonic Crystals - Ttwards rational material design," Nat. Mater. 2, 648-649 (2003). [CrossRef] [PubMed]
  6. M. F. Yanik, S. H. Fan, M. Soljacic, and J. D. Joannopoulos, "All-optical transistor action with bistable switching in a photonic crystal cross-waveguide geometry," Opt. Lett. 28, 2506-2508 (2003). [CrossRef] [PubMed]
  7. C. A. Barrios, V. R. Almeida, R. Panepucci, and M. Lipson, "Electrooptic modulation of silicon-on-insulator submicrometer-size waveguide devices," IEEE J. Lightwave Technol. 21, 2332-2339 (2003). [CrossRef]
  8. M. Law, D. J. Sirbuly, J. C. Johnson, J. Goldberger, R. J. Saykally, and P. D. Yang, "Nanoribbon waveguides for subwavelength photonics integration," Science 305, 1269-1273 (2004). [CrossRef] [PubMed]
  9. M. Ohtsu, K. Kobayashi, T. Kawazoe, S. Sangu, and T. Yatsui, "Nanophotonics: Design, fabrication, and operation of nanometric devices using optical near fields," IEEE J. Sel. Top. Quantum Electron. 8, 839-862 (2002). [CrossRef]
  10. J. F. Wang, M. S. Gudiksen, X. F. Duan, Y. Cui, and C. M. Lieber, "Highly polarized photoluminescence and photodetection from single indium phosphide nanowires," Science 293, 1455-1457 (2001). [CrossRef] [PubMed]
  11. O. Hayden, R. Agarwal, and C. M. Lieber, "Nanoscale avalanche photodiodes for highly sensitive and spatially resolved photon detection," Nat. Mater. 5, 352-356 (2006). [CrossRef] [PubMed]
  12. M. Freitag, Y. Martin, J. A. Misewich, R. Martel, and P. H. Avouris, "Photoconductivity of single carbon nanotubes," Nano Lett. 3, 1067-1071 (2003). [CrossRef]
  13. O. Astafiev, S. Komiyama, T. Kutsuwa, V. Antonov, Y. Kawaguchi, and K. Hirakawa, "Single-photon detector in the microwave range," Appl. Phys. Lett. 80, 4250-4252 (2002). [CrossRef]
  14. J. Alda, J. M. Rico-Garcia, J. M. Lopez-Alonso, and G. Boreman, "Optical antennas for nano-photonic applications," Nanotechnology 16, S230-S234 (2005). [CrossRef]
  15. K. T. Posani, V. Tripathi, S. Annamalai, N. R. Weisse-Bernstein, S. Krishna, R. Perahia, O. Crisafulli, and O. J. Painter, "Nanoscale quantum dot infrared sensors with photonic crystal cavity," Appl. Phys. Lett. 88, 151104, (2006). [CrossRef]
  16. A. P. Alivisatos, "Semiconductor clusters, nanocrystals, and quantum dots," Science 271, 933-937 (1996). [CrossRef]
  17. C. B. Murray, D. J. Norris, and M. G. Bawendi, "Synthesis and Characterization of Nearly Monodisperse Cde (e = S, Se, Te) Semiconductor Nanocrystallites," J. Am. Chem. Soc. 115, 8706-8715 (1993). [CrossRef]
  18. G. M. Whitesides, J. P. Mathias, and C. T. Seto, "Molecular Self-Assembly and Nanochemistry - A Chemical Strategy for the Synthesis of Nanostructures," Science 254, 1312-1319 (1991). [CrossRef] [PubMed]
  19. R. D. Schaller, M. Sykora, S. Jeong, and V. I. Klimov, "High-efficiency carrier multiplication and ultrafast charge separation in semiconductor nanocrystals studied via time-resolved photoluminescence," J. Phys. Chem. B 110, 25332-25338 (2006). [CrossRef] [PubMed]
  20. R. D. Schaller, M. Sykora, J. M. Pietryga, and V. I. Klimov, "Seven excitons at a cost of one: Redefining the limits for conversion efficiency of photons into charge carriers," Nano Lett. 6, 424-429 (2006). [CrossRef] [PubMed]
  21. Q3. E. J. Gansen, M. A. Rowe, M. B. Greene, D. Rosenberg, T. E. Harvey, M. Y. Su, R. H. Hadfield, S. W. Ham, and R. P. Mirin, "Photon-number-discriminating detection using a quantum-dot, optically gated, fieldeffect transistor," Nature Photonics 1, 585-588 (2007). [CrossRef]
  22. M. Drndic, M. V. Jarosz, N. Y. Morgan, M. A. Kastner, and M. G. Bawendi, "Transport properties of annealed CdSe colloidal nanocrystal solids," J. Appl. Phys. 92, 7498-7503 (2002). [CrossRef]
  23. N. Y. Morgan, C. A. Leatherdale, M. Drndic, M. V. Jarosz, M. A. Kastner, and M. Bawendi, "Electronic transport in films of colloidal CdSe nanocrystals," Phys. Rev. B 66, 075339, (2002). [CrossRef]
  24. G. Konstantatos, I. Howard, A. Fischer, S. Hoogland, J. Clifford, E. Klem, L. Levina, and E. H. Sargent, "Ultrasensitive solution-cast quantum dot photodetectors," Nature 442, 180-183 (2006). [CrossRef] [PubMed]
  25. Q4. G. Konstantatos, J. Clifford, L. Levina, and E. H. Sargent, "Sensitive solution-processed visible-wavelength photodetectors," Nat. Photonics 1, 531-534 (2007). [CrossRef]
  26. C. J. Wang, L. Huang, B. A. Parviz, and L. Y. Lin, "Subdiffraction photon guidance by quantum-dot cascades," Nano Lett. 6, 2549-2553 (2006). [CrossRef] [PubMed]
  27. D. Ginger and N. Greenham, "Electrical Properties of Semiconductor Nanocrystals," in Semiconductor and Metal Nanocrystals: Synthesis and Electronic and Optical Properties, V. I. Klimov, ed., (Marcel Dekker, Inc., New York, 2004), pp. 239-288.
  28. H. C. Liu and G. C. Aers, "Resonant tunneling through one-dimensional, two-dimensional, and three- dimensionally confined quantum wells," J. Appl. Phys. 65, 4908-4914 (1989). [CrossRef]
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