Ultrafast Emission Decay and Enhancement of ... - ACS Publications

Subscriber access provided by University of Newcastle, Australia

Article

Ultrafast Emission Decay and Enhancement of Blinking of Single Quantum Dots in the Presence of Silicon and Metal/Metal Oxide Structures Waylin J. Wing, and Seyed M. Sadeghi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11080 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Ultrafast Emission Decay and Enhancement of Blinking of Single Quantum Dots in the Presence of Silicon and Metal/Metal Oxide Structures Waylin J. Wing and Seyed M. Sadeghi

∗

Department of Physics, University of Alabama in Huntsville, Huntsville, Alabama, 35899, USA

E-mail: [email protected]

Abstract

Introduction

We experimentally investigate the impact of concentration of colloidal quantum dots on their lifetime and blinking dynamics when they are situated near material layers consisting of gold, Al oxide, and amorphous silicon combinations. Our results show that in the presence of the silicon layer the gold not only does not suppress the blinking behavior, but rather promotes it at much higher concentrations of the quantum dots. This process happens as the emission decay rate of the quantum dots is increased signicantly. For an amorphous silicon thickness of ∼5 nm, single quantum dots near the specic combination of gold/amorphous silicon/Al oxide layers exhibit mostly bright state behavior. However, when the thickness of the amorphous silicon is increased, we nd that the single quantum dots remain mostly in the dark state. The work presented here highlights the profound impact of defect sites in amorphous silicon on the interplay between plasmon eld enhancement and Auger recombination.The results can be used to modify and exploit desirable characteristics in the photoluminescent dynamics of colloidal quantum dots.

Quantum dots (QDs) are ideal entities for nanophotonic applications due to their many optical advantages such as narrow emission bandwidths, wide absorbance spectra, tunable resonance wavelength, and well-established fabrication techniques. 14 QDs embedded in polymer thin lms have been used to demonstrate a variety of such applications, including solar cells, 5 LED systems, 6,7 and photovoltaics. 8 An important parameter to consider when designing QD-based applications is the concentration of the QDs and how it impacts their photoluminescence and lifetime. 911 In structures such as a QD-polymer lm, the concentration can be correlated with the distance between QDs; high concentrations will lead to close proximity in the lm whereas low concentrations have increased distance between them. At very low concentrations, it is possible that the QDs be separated far enough from each other that they exhibit single QD-like behavior, such as intermittent photoluminescence (blinking). 1214 The lifetime and blinking behaviors of QDs are also heavily inuenced by the local environment of the QDs. In particular, a nearby metal surface can introduce nonradiative decay channels, resulting in a modication of the decay lifetime. 1522 For example, there can be surface energy transfer (SET) from QDs to the

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

plasmonic resonances in gold (Au) surfaces. 2326 Other metallic structures, such as nanoparticles, have been shown to reduce the lifetime when near a QD 2731 due to energy transfer to the localized surface plasmon resonance. Blinking dynamics of QDs have also been shown to change near a metal interface, where the most notable eect is the suppression of the blinking behavior of single QDs. 3134 Dielectrics such as silicon and metal oxides such as aluminum (Al) oxide are common materials used in nanophotonic and microelectronic applications. 35,36 These materials inuence QD photoluminescent behavior via the introduction of trap sites, for example. In a recent report, we demonstrated how an Al oxide layer deposited on top of a crystalline silicon substrate can increase the emission intensity of QDs over time via a eld-induced increase of photoinduced uorescent enhancement (PFE). 37 On the other hand, PFE can be signicantly suppressed due to a thin chromium oxide layer. 38 There have been studies that report on the electrostatic properties of QDs near dielectric and metal materials, 3941 but these reports do not take into account the blinking behavior or lifetime of the observed QDs. The choice of dielectric or metal oxide used in QD-based applications depends on the system requirements, and therefore it is imperative to understand how these materials interact with each other and the QDs in terms of the lifetime and emission dynamics. In this paper we experimentally study how thin layers of Au, amorphous silicon (am:Si), Al oxide, and combinations thereof, impact the lifetime and blinking dynamics of QDs in polymer lms as the concentration of QDs is altered. When QDs are positioned near Au/am:Si layers, the results suggest that in contrast to the common understanding, the Au not only does not suppress blinking, but rather promotes this process over prolonged periods of time at the high concentrations of the QDs. These results also show that such a process occurs as the lifetime of QDs is suppressed signicantly, reaching picosecond range. This phenomena occurs because the plasmonic eld of the Au enhances the radiative rate of the QDs, which then competes with the rates of the QD's

Page 2 of 16

electron photo-ejection to the surface defects and am:Si trap sites. Furthermore, the am:Si spacer is shown to inuence the blinking dynamics of the QDs, depending on its thickness. For a ∼5 nm am:Si layer, single QDs in the presence of Au/am:Si/Al oxide exhibit mostly bright states. However, this behavior can be reversed when the am:Si thickness is increased to ∼15 nm, yielding single QDs that remain mostly in the dark state. The variation of intensity of the QDs is also shown to be dependent on the am:Si thickness, where the thicker layer of 15 nm tends to decrease the variation. The results of this paper highlight the key importance of nanomaterial design and are applicable to nanophotonic applications where the lifetime and emission dynamics of QDs are desired to be controlled.

Experimental Methods The QDs used for this report are CdSe/ZnS (NN Labs, Inc.) and are diluted in a mixture of toluene and polymethylaccralate (PMMA). We use four concentrations of QDs: N1 (1.09 × 1011 QDs/µL), N2 (2.18 × 109 QDs/µL), N3 (1.09 × 109 QDs/µL), and N4 (1.82 × 108 QDs/µL). The material layer combinations of Au, am:Si, and Al oxide were fabricated onto a silica substrate by using a masking method and a sputter tool. For all experiments, the nominal thickness of the Au, am:Si, and Al oxide was 40 nm, 5 nm or 15 nm, and 1 nm, respectively. The QDs were spincoated onto the sample in order to measure the lifetime and blinking dynamics. In g.1a, the top and perspective view of the sample is shown, along with the corresponding regions containing the combination of material layers. Fig.1b shows a cross-sectional view of the sample structure with N1 and N4 concentrations, where N1 has close-packed QDs and N4 has well-separated QDs. To collect the emission spectra of the QDs, we use a setup in which a 514 nm laser passes through an objective and irradiates the sample at the focus of the objective. Light emitted from the excited QDs is collected via the same objective and sent back to a dichroic mirror,


2

is present (line 3), there is a large amount of SET, 2326 resulting in greatly reduced emission. The SiO2 /Au/Al oxide region (line 4) shows increased emission from the QDs, which might be due to the extra 1 nm of separation caused by the Al oxide layer as well as its passivating capabilities. We observe in g.2b that regions with combinations of am:Si/Al oxide, Au/am:Si, and Au/am:Si/Al oxide (lines 2, 3, and 4, respectively) have a signicantly reduced emission intensity as compared to the am:Si region (line 1). This is again because of the defects introduced by Al oxide and the SET to the Au. However, gs.2(a and b) suggest that the am:Si/Al oxide combination has more defects than the SiO 2 /Al oxide region, as evidenced by the dierence in emission amplitudes (lines 2).

Emission (a.u.)

12500

which in turn lters out the laser line while simultaneously sending the light from the QDs towards a cooled spectrometer. The lifetime of the QD emission is measured using Time Correlated Single Photon Counting (TCSPC) techniques, in which case the QDs are excited via a pulsed laser at 450 nm. Finally, images of the QDs are taken using an electron multiplying charged coupled device (EMCCD) camera while irradiated with a 514 nm laser.

(a)

1

Normalized counts (cps)

Figure 1: A top and perspective view of the sample structure is shown in (a), with the regions of various material layers labeled in the top view. In (b), a cross-sectional view highlighting the conceptual dierence between N1 and N4 concentrations is shown.

10000 7500

2

4

5000

3

2500 0 400

9000 Emission (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

600

(b)

700

800

1

6000 4500

4 2 3

1500 0 400

SiO2 (1) SiO2/Al (2) SiO2/Au (3)

0.75

SiO2/Au/Al (4)

1 0.5

2 3

0.25

900

7500

3000

(a')

1

4

0 0


Page 3 of 16

10

20

(b')

1

30

40

50

SiO2/Si (1) SiO2/Si/Al (2) SiO2/Au/Si (3)

1

0.75

SiO2/Au/Si/Al (4)

2 3

0.5

4

0.25 0

500 600 700 800 Wavelength (nm)

900

0

10

20 30 Time (ns)

40

50

Figure 2: The emission spectra of the QDs on the four regions is shown in (a) for the sample without am:Si, and the corresponding lifetime is shown in (a0 ). Likewise, in (b) the emission spectra is shown for samples with 5 nm of am:Si, and the lifetime is displayed in (b 0 ).

Results The emission of QDs is quite dierent on 5 nm of am:Si than on the silica (SiO 2 ) substrate, as shown in g.2 for the N1 concentration. In g.2a we see the emission of the QDs on a sample without the am:Si layer. This sample was fabricated by depositing the Au and Al oxide layers directly onto the SiO 2 substrate. The emission is the highest when directly on the SiO2 (line 1). The addition of Al oxide reduces the emission as seen by line 2. Although Al oxide has been shown to passivate surface defects of underlying substrates, 42,43 it also introduces its own defects which can be structural or electronic in nature. 4446 When Au

The dierences in lifetime between SiO 2 and am:Si samples is shown in gs.2(a 0 and b0 ), respectively. In g.2a 0 we see that the QDs located directly on the SiO 2 (line 1) have the longest lifetime, and that QDs on SiO 2 /Al oxide have a similar but slightly shortened lifetime. This is compatible with the spectra seen in g.2a, where the emission of QDs on the SiO2 /Al oxide region is slightly less due to the introduction of defects. These defects increase the eciency of nonradiative decay to trap (intermediate) states, resulting in shorter lifetime


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in the SiO2 /Al oxide region, suggesting that Al oxide does not oer much impact on the QD lifetime when used in conjunction with SiO 2 . For regions with an Au layer, both the τf and τs are much faster than when Au is absent. This is due to the ecient decay channels oered up by the SET from QDs to the Au layer. The τf , τs , and amplitude ratio trends are drastically dierent when 5 nm of am:Si is present, as shown in g.3. The normalized decay of the QDs for all four concentrations is shown in g.3(a-d) for am:Si, am:Si/Al oxide, Au/am:Si, and Au/am:Si/Al oxide, respectively. In g.3(a0 -d0 ), the τs , τf , and amplitude ratio for these regions are shown as a function of concentration. On the am:Si region, we note that the τs increases from 24 ns to 60 ns when the concentration decreases from N1 to N4. The τf decreases from 5.8 ns to 0.8 ns over the same QD concentration decrease. The amplitude ratio decreases from 0.9 to 0.7 from N1 to N2, remains close to 0.7 at N3, and then rapidly increases to 1.8 as the concentration decreases to N4. This means that the τs contributed slightly more up until N3, at which point the τf begins to be the main contributor to the decay curve seen in g.3a (black curve). The shortened decay lifetime as N1 goes to N4 could be because as the concentration decreases to N4, more of the QDs are near the surface and thus the surface defect sites. Furthermore, as the concentration decreases to N4, the density of defect sites available to the QDs increases, further shortening the lifetime. The QD lifetimes and amplitude ratio for the am:Si/Al oxide region are shown in g.3b 0 . The τs increases from 15 ns to 99 ns as the concentration decreases, and the τf increases from 2.5 ns to 3.3 ns going from N1 to N3. At N4, the τf decreases to 1.5 ns. The amplitude ratio is 7.9 for the N1 concentration, and decreases as the concentration decreases to 1.8 at N4. The decrease in amplitude ratio with decreasing concentration suggests that less defect sites are available to the QDs. This highlights the advantageous properties of Al oxide, namely its passivating eect on the am:Si and the prevention of electron ejection to the am:Si via the eld induced by its large negative xed charge.

and decreased emission intensity. We observe that the lifetimes on SiO 2 /Au and SiO2 /Au/Al oxide (lines 3 and 4, respectively, in g.2a 0 ) are drastically reduced due to the ecient SET from the QDs to the Au. The SET leads to a dominant decay channel and thus very rapid lifetime and reduced emission intensity. In g.2b0 , we see that the am:Si region has the longest lifetime, and the other regions are much faster. Like the SiO 2 sample, the short lifetime of Au/am:Si and Au/am:Si/Al oxide regions is most likely due to nonradiative SET. There is no Au present in the am:Si/Al oxide region, and thus the lifetime becomes slightly longer. The shorter lifetime of am:Si/Al oxide region compared to the am:Si region is because the addition of the Al oxide layer introduces surface defects to the region as mentioned previously. The relatively shorter lifetime of am:Si/Al oxide compared to SiO2 /Al oxide (lines 2 of gs.2a 0 and b0 ) again suggests that the combination of am:Si/Al oxide introduces more defect sites than SiO2 /Al oxide. We now investigate how the concentration of QDs impacts their lifetime when spincoated onto samples without am:Si (Au and Al oxide fabricated directly on SiO 2 substrate). For this, we measure and normalize the decay on each region for all four concentrations, and t the data to a bi-exponential curve given by: 47,48

y(t) = Ae

− τt

f

t

+ Be− τs ,

Page 4 of 16

(1)

where A is the amplitude of the fast decay time contribution to average lifetime, τf is the fast decay time, B is the amplitude of the slow decay time contribution to average lifetime, and τs is the slow decay time. We note that the amplitude ratio (A/B ) tells us how much the slow or fast lifetime contributes to the bi-exponential decay curve. The results for the τf , τs , and amplitude ratio (Supporting Information, g.S1) indicate that as the concentration of QDs decreases in the SiO2 region, the amplitude ratio and τf remains mostly constant but the τs greatly increases. This means that the radiative emission eciency decreases as the concentration decreases on the SiO2 . We observed very similar trends


4

0.75

(b) am:Si/Al

1

N1 N2 N3 N4

(c) Au/am:Si

1

0.75

0.75

0.75

0.5

0.5

0.5

0.5

0.25

0.25

0.25

0.25

0

0 0

10

20

30

40

50

0 0

10

Time (ns) 60 τs (1)

50

τf (2)

40

50

0 0

10

Time (ns) 10

100

8

20

30

40

50

0

16

80

8

12

6

60

6

4

40

2

20

(b')

(c')

10

16

8

12

0 N1

N2N3

Concentration

0 N4

1

N2N3

40

(d')

0 N4

Concentration

2

2

0 N1

50

10 8

1

6

4 4

2

0 N1

30

8

4

1 2

20

Time (ns)

6 8

2

10

Time (ns) 10

1

20 10

30

A/B ratio

40 30

(a')

20

(d) Au/am:Si/Al

1

N2N3

Concentration

0 N4

4 4

2

0 N1

2

N2N3

Amplitude ratio (A/B)

(a) am:Si

1

Lifetime (ns)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 5 of 16

0 N4

Concentration

Figure 3: Lifetime measurements of the QDs for concentrations N1 through N4 are shown for the am:Si region (a), am:Si/Al oxide region (b), Au/am:Si region (c), and Au/am:Si/Al oxide region (d). The τf , τs , and the ratio of amplitude of A/B are shown in (a0 -d0 ) for the am:Si, am:Si/Al oxide, Au/am:Si, and Au/am:Si/Al oxide regions, respectively, as a function of the concentrations N1 through N4. In g.3c0 for the Au/am:Si region, we note that the τs increases from 6 ns to 14 ns as the QD concentration drops from N1 to N4. The τf also increases slightly as the concentration decreases from N1 to N3, going from 1.0 ns to about 1.7 ns. At the most dilute concentration of N4, the τf decreases to 1.3 ns, slightly higher than that of N1. The amplitude ratio at N1 is 10.1, and drops o to about 4.8 at N3. When the concentration is decreased further to N4, the amplitude ratio increases to 5.3. We note that the main dierence when Au is present is that the lifetimes of both τf and τs are much faster (less than 15 ns), and that the τf contribution is much higher. At N4, the amplitude ratio is more than double that of am:Si or am:Si/Al oxide. Finally, we measure the QD lifetime of N1N4 on the Au/am:Si/Al oxide region. The τf , τs , and amplitude ratio parameters are shown in g.3d0 . The τs increases from 8 ns to 15 ns as the concentration decreases from N1 to N4. The τf remains fairly constant over the same concentration decrease, going from 1.4 ns at N1 to 1.5 ns at N4. The amplitude ratio starts at about 9.2 and decreases to 7.0 at N3, at which

point it begins to increase slowly and is 7.4 at N4. When compared to the Au/am:Si region, it's observed that the addition of Al oxide serves to keep the τf mostly xed at a constant value, while slightly increasing the τs (by 2 ns at N1 and 1 ns at N4). Furthermore, the Al oxide seems to increase the amplitude ratio at concentrations of N2 - N4, meaning that it increases the contribution of τf . The plots of g.3 lend us some noteworthy trends. First, as the concentration decreases from N1 to N4, the τs always increases, independent of which region the QDs were measured on. However, this increase in τs varies depending on the region. For example, τs increases by 6.4 times for the am:Si/Al oxide region going from N1 to N4, but only by 1.9 times for the Au/am:Si/Al oxide region. Another trend is that when Au is present, both the τf and the τs are greatly reduced. This is due to the ecient SET from the excited QDs to the Au plasmons, leading to a reduction in lifetime. The plots in g.3 also demonstrate some contrasting physics between the lifetimes of the QDs in the dierent regions. The most prominent case is between the am:Si and am:Si/Al


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

oxide layers. For am:Si, we see that the τs increases from ∼24 ns to 60 ns over the concentrations studied. In contrast, the τs for am:Si/Al oxide increases from ∼15 ns to 99 ns. Furthermore, the amplitude ratio of am:Si remains nearly unchanged from N1 to N4, whereas for am:Si/Al oxide, the amplitude ratio starts high at ∼8 and decreases to 2 at N4. This suggests that Al oxide causes the bi-exponential lifetime of the QDs to have mostly fast-decay character at low concentrations, but to gradually become slow-decay dominant at low concentrations. At N4, the amplitude ratio of am:Si and am:Si/Al oxide are nearly the same. This, coupled with the long (99 ns) τs of the am:Si/Al oxide QDs at N4, could mean that QDs near a thin Al oxide layer are less ecient emitters at single QD concentrations than for am:Si regions. This highlights the impact of the material layer structure on the QD dynamics, and suggests that even a very thin layer of Al oxide can dramatically alter the QD photoluminescent properties. We then move on to explore how the material layer structure impacts the blinking dynamics of the QDs as the concentration transitions from a lm with close-packed QDs (N1) to a lm with well-spaced QDs (N4). For this, we use a sensitive EMCCD camera to capture timelapsed images of the sample regions. The QDs are imaged by the EMCCD camera for 400 secs, and their behavior over this time period is discerned by observing their intensity from the images. The exposure time (or bin time) for each frame is 1.0 sec. For a fair analysis, 40 points or spots on the image are analyzed in terms of their intensity behavior. For a complete description of the EMCCD imaging setup and image analysis, see Supporting Information, Section S1 and Fig.S2. The rst image in the time-lapse measurement for all four regions is shown in g.4(a-d), for N1 - N4 concentrations, respectively. The spot circled in red in each image corresponds to the spot with the highest coecient of variation (Supporting Information, Section 1) of the 40 chosen spots. In g.4a for the N1 concentration we observe that the am:Si and am:Si/Al oxide regions appear to have features that might be agglomeration of QDs, which gives a rough,

Page 6 of 16

textured aspect to the image. This is indicative that the QDs are packed close to each other. On the other hand, for the Au/am:Si and Au/am:Si/Al oxide regions, it appears as though the dominant features are several bright spots, which are well-spaced from each other. In between these bright spots, there is little background intensity or other features, as seen in the am:Si and am:Si/Al oxide images. These bright spots are not single QDs, but most likely enhanced and agglomerated QDs. These QDs could be located near top of the QD-polymer lm, suciently far enough that their emission can be enhanced by the Au. 18,49,50 The enhanced QDs appear even brighter when they are agglomerated together. In g.4a 0 , the intensities of the circled spots shown in g.4a are plotted for each region as a function of time. Here we see slight uctuation in intensity for each spot and slight enhancement of emission in the am:Si and am:Si/Al oxide regions which might be due to PFE. QDs that exhibit PFE do so because of the Coulomb blockade created by ionized QDs. Ionized QDs form a eld barrier, which prevents nearby QDs from becoming ionized, thus staying in the bright, emissive state. Over time, as more QDs become ionized and thereby preventing nearby QDs from becoming ionized, the observed emission of the QDs increases, thus marking PFE. The PFE that we do observe in am:Si and am:Si/Al oxide regions of N1 could be due to the eld passivation eect of Al oxide at the am:Si/Al oxide interface 37 and by the high concentration where the Coulomb blockade has the most impact on proximal QDs. The N2 concentration images and intensity plot are shown in g.4(b and b 0 ), respectively. We see here that there is slightly more variation across the images of the am:Si and am:Si/Al oxide regions, meaning that the QDs have become slightly more separated from each other. The Au/am:Si and Au/am:Si/Al oxide regions look similar to the N1 case, in which a reduced background intensity and bright spots are observed. The intensity plots shown in g.4b 0 are dramatically dierent than for the N1 concentration. Here, we see a much larger uctuation for all regions, and for regions with Au,


6

Page 7 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: The rst image of the time lapse for each region of concentrations N1 - N4 are shown in (a-d), respectively. The red circle represents the point on the image with the highest coecient of variation of 40 analyzed points. In (a 0 -d0 ), the intensity of the circled spot is plotted as a function of time. In (a0 -d0 ), line 1 corresponds to am:Si, line 2 to am:Si/Al oxide, line 3 to Au/am:Si, and line 4 to Au/am:Si/Al oxide. it appears that there is some blinking behavior. There is a bright and dark state for spots located on the Au/am:Si and Au/am:Si/Al oxide regions, as indicated by prolonged intensities that are either low or high compared to the average intensity. We note that at this concentration we do not expect the QDs to be separated far enough to exhibit single QD behavior, even though we do observe this in g.4b 0 . This could be indicative of a unique process for these regions, e.g., the competing decay rates of the QDs due to am:Si and Au layers, which will be discussed shortly. We see the results of the N3 concentration images in g.4c. We can begin to see the illumination from the QDs as circular bright spots in the image, although they are still very close to each other. In the Au/am:Si and Au/am:Si/Al oxide regions, we still observe the reduction of the background intensity, along with several bright spots. Like the N2 case, there is blinking behavior for regions when Au is present, as indicated by large uctuations in the intensity over short

periods of time and by prolonged bright/dark states. We note that the Au/am:Si/Al oxide region exhibits long periods of time in the dark state, and that periods in the bright state are relatively short-lived (g.4c 0 , line 4). When we decrease the concentration of QDs to N4, there is drastic change in the images and intensity plots as shown in gs.4(d and d0 ). Here we observe from the images that the QDs in the am:Si and am:Si/Al oxide regions are now well-spaced from each other, and individual QDs can be distinguished. This is the single QD limit for the Si region, where the intensity plot (g.4d0 ) of this concentration show that there is blinking of QDs. We note the reversal of behavior for the Au/am:Si/Al oxide region (g.4d0 , line 4) as compared to g.4c 0 , where now the QD seems to be mostly in the bright state, and that the dark state is shortlived and uncommon. This reversal behavior seen when comparing lines 4 in g.4c 0 and 4d0 is most likely due to the statistical nature of the QDs and not due to, for example, a change in


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 16

Discussion

environment. This will be discussed in the next section. To understand how the separation between the QDs and the Au layer impacts the blinking dynamics, we fabricated identical samples except with an am:Si thickness of 15 nm instead of 5 nm. The image results for N1 (Supporting Information, gure S3) show us that the quenching of QD emission is reduced near Au layers and the intensity plots of N2 and N3 (Supporting Information, gure S4) indicate that blinking behavior occurs on all four regions. The EMCCD images for the N4 concentration on this sample is shown in g.5, along with the intensity plot. The most striking feature is that the QDs on the Au/am:Si/Al oxide region remain mostly in the dark state (line 4). This is in contrast to the N4 concentration of the 5 nm am:Si sample, where the QD was observed to be mostly in a bright state (g.4d 0 , line 4). We also observe that the intensity of the am:Si/Al oxide region spends less time in the dark state as compared to the am:Si region, which could be due to the prevention of carrier decay to the defects in am:Si by the charge barrier created by the thin Al oxide layer.

One new result presented by this paper is the very short lifetime of single QDs deposited on the 5 nm am:Si layer (g.3a). This short lifetime can be attributed to the fact that an increased number of QDs are in contact with the am:Si surface, and that the density of defect sites is increased, all because the concentration of QDs is dilute. Short lifetimes such as that seen in g.3a for N4 are usually associated with QDs near metal or close-packed such that FRET is dominant, for example. This short lifetime is not observed for the SiO 2 case at N4 (Supporting Information, gure S1), suggesting that there are many more defect sites in am:Si than for the SiO2 substrate. Another key result is the apparent blinking of some QDs at high concentrations when near Au with a 5 nm am:Si spacer. We believe that the observation of this blinking is due to the enhancement of the radiative decay rate in conjunction with the exposure time (or bin time) of our EMCCD camera. To discuss this, we qualitatively describe blinking in terms of the QD and its environment. Although the exact mechanism is unknown, the general consensus is that blinking occurs in QDs due to the charging and re-neutralization of the QD, 14,51 whereby the charge carriers may be delocalized via processes such as Auger ionization or quantum mechanical tunneling to surface defects. Since, for example, the delocalized electron cannot contribute to recombination, the QD does not emit (becomes dark) until an electron re-enters the excited state and recombines. This is shown in (i) of g.6, where the QD is considered to be a two-level system in vacuum. The transition from |2i to |1i represents recombination and radiative emission. Here, ionization of the QD occurs through tunneling to surface defects. The only decay rates are radiative decay ( krad ) and the decay to the surface defects ( ksur ). When the am:Si layer is present near the QDs, another decay rate ( kam:Si ) appears that is due to the ejection of electrons to the numerous defect sites present in the am:Si as shown in (ii) in g.6. This kam:Si competes with the krad and ksur . The kam:Si can cause the emission and

Figure 5: The rst image in the time-lapse measurements for each region of the 15 nm am:Si sample with the N4 concentration. The intensity as a function of time for the circled spots is shown below. Line 1 corresponds to am:Si, line 2 to am:Si/Al oxide, line 3 to Au/am:Si, and line 4 to Au/am:Si/Al oxide.


8

Page 9 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dark states are discernable because the suppression causes the QDs to stay in the bright state for longer periods of time. This is in contrast to the am:Si region where the fast electron kinetics can lead to random uctuation in the observed emission. This phenomena is shown in g.4b 0 , for both the am:Si (line 1) and the Au/am:Si (line 3) regions, where for Au/am:Si the bright and dark states can be distinguished. We also nd that by increasing the am:Si thickness to 15 nm, the bright state of single QDs on the Au/am:Si/Al oxide region (g.4d 0 , line 4) can be made to be mostly in the dark state (g.5, line 4), for the same concentration (N4). In this case, this could be due to the increased number of trap states available to the QDs as compared to the 5 nm am:Si sample. To further elucidate how the material layer structure and concentration impact the photoluminescent properties of the QDs, the standard deviation (σ ) of the intensity and the average (µ) intensity of the 40 spots we chose in the EMCCD images are calculated and shown in g.7. In g.7(a-d), the spots for the 5 nm am:Si sample are plotted for the am:Si, am:Si/Al oxide, Au/am:Si, and Au/am:Si/Al oxide regions, respectively. We note a few general trends. First, for the N1 concentration, there is a very good correlation between σ and µ of the chosen spots for all four regions. This means that the amount of uctuation in the intensity increases linearly as the average intensity of the QD increases. Second, for the N2 - N4 concentrations, we note that the correlation between σ and µ becomes weaker as the concentration decreases. Especially for the N4 concentration, the correlation is very weak and most of the spots lie near the origin of the plots. This suggests that as the single QD regime is approached, the blinking dynamics of the QDs are mostly independent of their intensity, and that the observed QDs become similar in their blinking dynamics. We specically note that for the am:Si/Al oxide region (g.7b), the QD values become more compact and restricted as the concentration decreases. Here, for N1 the values of σ and µ span a large range. Going from N2 to N4, the spread of QD values decreases until at N4 all 40 QDs are extremely similar in their σ and µ, which

Figure 6: A QD modeled as a two level system is shown in vacuum (i), near am:Si (ii), and near Au/am:Si (iii). the lifetime of the QD to be reduced, as seen in g.2(b and b0 ), respectively. In the structures studied here, we assume that the electron can return quickly to the QD from the am:Si because of the band alignment of the QD and am:Si. 52,53 This alignment allows electrons that have been transferred to the am:Si to be able to return back to the QD, thus neutralizing it. In this paper, we show that the underlying Au layer leads to blinking in the N2 concentration when compared to regions without Au. This blinking, however, is actually due to the suppression of fast blinking dynamics in the QDs which is itself caused by the ejection of an electron to the am:Si and the subsequent fast return to the QD. We believe the suppression of blinking is because of the enhancement of krad of the QDs. In other studies, the blinking of QDs was shown to be suppressed due to the presence of Au. 31,33 This suppression in blinking could be correlated with an enhanced krad , which was near or higher than the competing nonradiative decay rates, thus leading to long durations of bright QDs. We believe the same enhancement of krad happens in our samples, but in the presence of am:Si ( kam:Si ) and ksur the enhancement is not large enough to completely keep the QDs in a bright state over the time scale studied here. The bin time of each data point collected in the intensity plots (1 sec) prevented us from observing the fast blinking dynamics. However, since this fast blinking is suppressed in regions with Au, the bright and


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

again suggests they have very similar blinking dynamics. Fig.7(a-d) also gives us insight into the intensity plots of g.4(a0 -d0 ). The intensity plots featured QDs with the highest coecient of variation, which is simply the σ divided by the µ. We can observe in g.7(a-d) that in some regions and concentrations, the distribution of the QD values can span large ranges and that there exists outliers. In choosing the QDs to plot with the highest coecient of variation, it's possible that the intensity plots might not be representative of the entire population. This statistical nature of the QD emission intensity could be the cause of the reversal behavior we observed in lines 4 of g.4c0 and 4d0 . In g.7(a0 -d0 ), we plot the σ and µ of the four regions and concentrations for the 15 nm am:Si sample. We again observe a relationship between σ and µ for the N1 concentration for all four regions. We note that the values of σ are smaller in g.7(a0 -d0 ) than in g.7(a-d), which means that the emission of the QDs did not uctuate as much in 15 nm am:Si samples as 5 nm am:Si samples. The main dierence between 5 nm am:Si and 15 nm am:Si samples is that for 15 nm am:Si (g.7(a0 -d0 )), it appears that the material layer structure does not have as large an inuence on the σ and µ. We observe that for all four regions, the grouping of the spots are relatively the same in relation to each other. This is in contrast to the 5 nm am:Si samples (g.7(a-d)), in which case the grouping of the spots is much dierent when the regions are compared. This dierence might be expected in regions where Au is present, since the distance between the QDs and the Au plays a large role in determining the QD lifetime and photoluminescent properties. If the QDs are farther away from the Au as in the 15 nm am:Si sample, then the Au will have less impact and the regions might look similar. However, even for am:Si and am:Si/Al oxide regions, the 10 nm dierence between the samples seems to change the distribution of the spots. This highlights that not only is the material composition important, but also the thickness of the material layers that make up the comprehensive structure.

Page 10 of 16

The results of this paper highlight how one can control the emission dynamics of QDs using the correlated impact of Au thin lms and defect sites in am:Si layers. These results also show the way QD concentration can inuence such dynamics, oering new insight into ultrafast ejection of photo-excited electrons from QDs to such defect sites and their return back to the QDs. The alteration of the decay times can be important for many QD-based applications where the lifetime needs to be controlled. 54,55 The ultrafast decay supported by the combined eects of Au and am:Si can be particularly useful for fast optical QD-based devices. The blinking dynamics studied in this paper show the interplay between QD concentration and the impact of dielectric and metals. This can be benecial for the design of devices that use thin lms of QDs and need control over their emissive and lifetime properties, especially as in solar cells applications. 56

Conclusion In conclusion, we have investigated how thin layers of am:Si, am:Si/Al oxide, Au/am:Si, and Au/am:Si/Al oxide impact the lifetime and blinking of QDs when the concentration of QDs in QD-polymer spincoated lms is altered. We showed that Au can lead to an enhanced radiative decay rate in the QDs, which can lead to blinking at high concentrations. This process happens simultaneously with an ultrafast decay of the QDs. We found that by adding Al oxide to these structures, the QDs can be made to emit in either a bright state or remain dark, depending on the concentration of QDs. Furthermore, we found that increasing the am:Si thickness to 15 nm results in blinking behavior for QDs in all regions of the sample, and that the previously bright state of single QDs was reversed to a dark state in the 15 nm samples. Finally, we also nd that the distribution of variation in QD intensities is dierent considering the two studied thicknesses of am:Si, with thicker am:Si resulting in less variation in intensity.


10

Page 11 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7: Standard deviation of intensity of the 40 spots as a function of the average intensity, for the studied concentrations and regions of the 5 nm (left) and 15 nm (right) am:Si samples. The am:Si (a, a0 ), am:Si/Al oxide (b, b 0 ), Au/am:Si (c, c0 ), and Au/am:Si/Al oxide (d, d 0 ) regions are shown for 5 nm and 15 nm am:Si, respectively.

Supporting Information Avail-

man, E. R.; Mattoussi, H. Quantum Dot Bioconjugates for Imaging, Labelling and Sensing. Nat. Mater. 2005, 4, 435446.

able The following les are available free of charge. Fig.S1: SiO2 lifetimes and amplitude ratio. Fig.S2: EMCCD experimental setup. Fig.S3: Images of N1 on the 15 nm am:Si sample. Fig.S4: Intensity plots of N2 and N3 concentrations for 15 nm am:Si sample. Section 1: EMMCD setup description and image analysis.

(4) Kim, J. Y.; Voznyy, O.; Zhitomirsky, D.; Sargent, E. H. 25th Anniversary Article: Colloidal Quantum Dot Materials and Devices: A Quarter-Century of Advances. Adv. Mater. 2013, 25, 49865010. (5) Choi, J. J.; Wenger, W. N.; Homan, R. S.; Lim, Y.-F.; Luria, J.; Jasieniak, J.; Marohn, J. A.; Hanrath, T. Solution-Processed Nanocrystal Quantum Dot Tandem Solar Cells. Adv. Mater. 2011, 23, 31443148.

Acknowledgement This work is supported

by US National Science Foundation under grant no. CMMI 1234823 and UAH Individual Investigator Distinguished Research award.

(6) Sun, L.; Choi, J. J.; Stachnik, D.; Bartnik, A. C.; Hyun, B.-R.; Malliaras, G. G.; Hanrath, T.; Wise, F. W. Bright Infrared Quantum-Dot Light-Emitting Diodes Through Inter-Dot Spacing Control. Nat. Nanotechnol. 2012, 7, 369373.

References (1) Murphy, C. J. Optical Sensing with Quantum Dots. Anal. Chem. 2002, 74, 520A. (2) Bailey, R. E.; Smith, A. M.; Nie, S. Quantum Dots in Biology and Medicine.

(7) Kwak, J.; Bae, W. K.; Zorn, M.; Woo, H.; Yoon, H.; Lim, J.; Kang, S. W.; Weber, S.; Butt, H.-J.; Zentel, R. et al. Characterization of Quantum Dot/Conducting Poly-

Physica E: Low-dimensional Systems and Nanostructures

2004, 25, 112.

(3) Medintz, I. L.; Uyeda, H. T.; Gold-


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

mer Hybrid Films and Their Application to Light-Emitting Diodes. Adv. Mater. 2009, 21, 50225026.

tals. J. 1678.

Opt. Soc. Am. B

Page 12 of 16

2006, 23, 1674

(16) Matsuda, K.; Ito, Y.; Kanemitsu, Y. Photoluminescence Enhancement and Quenching of Single CdSe/ZnS Nanocrystals on Metal Surfaces Dominated by Plasmon Resonant Energy Transfer. Appl. Phys. Lett. 2008, 92, 211911.

(8) Koleilat, G. I.; Levina, L.; Shukla, H.; Myrskog, S. H.; Hinds, S.; PattantyusAbraham, A. G.; Sargent, E. H. Ecient, Stable Infrared Photovoltaics Based on Solution-Cast Colloidal Quantum Dots. ACS Nano 2008, 2, 833840.

(17) Ito, Y.; Matsuda, K.; Kanemitsu, Y. Mechanism of Photoluminescence Enhancement in Single Semiconductor Nanocrystals on Metal Surfaces. Phys. Rev. B 2007, 75, 033309.

(9) Mei, B. C.; Wang, J.; Qiu, Q.; Heckler, T.; Petrou, A.; Mountziaris, T. Dilution Eects on the Photoluminescence of ZnSe Quantum-Dot Dispersions. Appl. Phys. Lett. 2008, 93, 083114.

(18) Shimizu, K.; Woo, W.; Fisher, B.; Eisler, H.; Bawendi, M. Surface-Enhanced Emission from Single Semiconductor Nanocrystals. Phys. Rev. Lett. 2002, 89, 117401.

(10) Droseros, N.; Seintis, K.; Fakis, M.; Gardelis, S.; Nassiopoulou, A. G. Steady State and Time Resolved Photoluminescence Properties of CuInS 2/ZnS Quantum Dots in Solutions and in Solid Films. J. Lumin. 2015, 167, 333338.

(19) Ueda, A.; Tayagaki, T.; Kanemitsu, Y. Energy Transfer from Semiconductor Nanocrystal Monolayers to Metal Surfaces Revealed by Time-Resolved Photoluminescence Spectroscopy. Appl. Phys. Lett. 2008, 92, 133118.

(11) Hartmann, L.; Kumar, A.; Welker, M.; Fiore, A.; Julien-Rabant, C.; Gromova, M.; Bardet, M.; Reiss, P.; Baxter, P. N.; Chandezon, F. et al. Quenching Dynamics in CdSe Nanoparticles: Surface-Induced Defects Upon Dilution. ACS Nano 2012, 6, 90339041.

(20) Zhai, Y.-Y.; Song, H.; Zhou, Z.-K.; Li, M.; Hao, Z.-H. Enhanced Radiative Emission Rate of CdSe/ZnS Quantum Dots on Semi-Continuous Gold Films. Eur. Phys. J. Appl. Phys. 2008, 42, 109112.

(12) Frantsuzov, P.; Kuno, M.; Janko, B.; Marcus, R. A. Universal Emission Intermittency in Quantum Dots, Nanorods and Nanowires. Nat. Phys. 2008, 4, 519522.

(21) Wu, X.; Sun, Y.; Pelton, M. Recombination Rates for Single Colloidal Quantum Dots Near a Smooth Metal Film. Phys. Chem. Chem. Phys. 2009, 11, 58675870.

(13) Galland, C.; Ghosh, Y.; Steinbrück, A.; Sykora, M.; Hollingsworth, J. A.; Klimov, V. I.; Htoon, H. Two Types of Luminescence Blinking Revealed by Spectroelectrochemistry of Single Quantum Dots. Nat. 2011, 479, 203207.

(22) Hartseld, T.; Gegg, M.; Su, P.-H.; Buck, M.; Hollingsworth, J. A.; Shih, C.K.; Richter, M.; Htoon, H.; Li, X. Semiconductor Quantum Dot Lifetime Near an Atomically Smooth Ag Film Exhibits a Narrow Distribution. ACS Photon. 2016,

(14) Efros, A. L.; Nesbitt, D. J. Origin and Control of Blinking in Quantum Dots. Nat. Nanotechnol. 2016, 11, 661671.

(23) Weber, W.; Eagen, C. Energy Transfer from an Excited Dye Molecule to the Surface Plasmons of an Adjacent Metal. Opt. Lett. 1979, 4, 236238.

(15) Okamoto, K.; Vyawahare, S.; Scherer, A. Surface-Plasmon Enhanced Bright Emission from CdSe Quantum-Dot Nanocrys-


12

Page 13 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Campion, A.; Gallo, A.; Harris, C.; Robota, H.; Whitmore, P. Electronic Energy Transfer to Metal Surfaces: a Test of Classical Image Dipole Theory at Short Distances. Chem. Phys. Lett. 1980, 73, 447 450.

(32) Fu, Y.; Zhang, J.; Lakowicz, J. R. Suppressed Blinking in Single Quantum Dots (QDs) Immobilized Near Silver Island Films (SIFs). Chem. Phys. Lett. 2007, 447, 96100. (33) Ma, X.; Tan, H.; Kipp, T.; Mews, A. Fluorescence Enhancement, Blinking Suppression, and Gray States of Individual Semiconductor Nanocrystals Close to Gold Nanoparticles. Nano Lett. 2010, 10, 4166 4174, PMID: 20825166.

(25) Pockrand, I.; Brillante, A.; Möbius, D. Nonradiative Decay of Excited Molecules Near a Metal Surface. Chem. Phys. Lett. 1980, 69, 499504. (26) Persson, B.; Lang, N. Electron-Hole-Pair Quenching of Excited States Near a Metal. Phys. Rev. B 1982, 26, 5409.

(34) Ji, B.; Giovanelli, E.; Habert, B.; Spinicelli, P.; Nasilowski, M.; Xu, X.; Lequeux, N.; Hugonin, J.-P.; Marquier, F.; Greet, J.-J. et al. Non-Blinking Quantum Dot with a Plasmonic Nanoshell Resonator. Nat. Nanotechnol. 2015, 10, 170 175.

(27) Yuan, C. T.; Yu, P.; Ko, H. C.; Huang, J.; Tang, J. Antibunching Single-Photon Emission and Blinking Suppression of CdSe/ZnS Quantum Dots. ACS Nano 2009, 3, 30513056, PMID: 19856980.

(35) Cho, E.-C.; Green, M. A.; Conibeer, G.; Song, D.; Cho, Y.-H.; Scardera, G.; Huang, S.; Park, S.; Hao, X.; Huang, Y. et al. Silicon Quantum Dots in a Dielectric Matrix for All-Silicon Tandem Solar Cells. Adv. OptoElectron. 2007, 2007 .

(28) Strelow, C.; Theuerholz, T. S.; Schmidtke, C.; Richter, M.; Merkl, J.P.; Kloust, H.; Ye, Z.; Weller, H.; Heinz, T. F.; Knorr, A. et al. Metal Semiconductor Nanoparticle Hybrids Formed by Self-Organization: A Platform to Address ExcitonPlasmon Coupling. Nano Lett. 2016, 16, 48114818.

(36) Wood, V.; Panzer, M.; Halpert, J.; Caruge, J.-M.; Bawendi, M.; Bulovic, V. Selection of Metal Oxide Charge Transport Layers for Colloidal Quantum Dot LEDs. ACS Nano 2009, 3, 35813586.

(29) Ratchford, D.; Shaei, F.; Kim, S.; Gray, S. K.; Li, X. Manipulating Coupling Between a Single Semiconductor Quantum Dot and Single Gold Nanoparticle. Nano Lett. 2011, 11, 10491054.

(37) Patty, K.; Sadeghi, S.; Nejat, A.; Mao, C. Enhancement of Emission Eciency of Colloidal CdSe Quantum Dots on Silicon Substrate via an Ultra-Thin Layer of Aluminum Oxide. Nanotechnol. 2014, 25, 155701.

(30) Cohen-Hoshen, E.; Bryant, G. W.; Pinkas, I.; Sperling, J.; Bar-Joseph, I. ExcitonPlasmon Interactions in Quantum DotGold Nanoparticle Structures. Nano Lett. 2012, 12, 42604264.

(38) Sadeghi, S.; Nejat, A.; Weimer, J.; Alipour, G. Chromium-Oxide Enhancement of Photo-Oxidation of CdSe/ZnS Quantum Dot Solids. J. Appl. Phys. 2012, 111, 084308.

(31) Naiki, H.; Masuo, S.; Machida, S.; Itaya, A. Single-Photon Emission Behavior of Isolated CdSe/ZnS Quantum Dots Interacting with the Localized Surface Plasmon Resonance of Silver Nanoparticles. J. Phys. Chem. C 2011, 115, 23299 23304.

(39) Krauss, T. D.; Brus, L. E. Charge, Polarizability, and Photoionization of Single Semiconductor Nanocrystals. Phys. Rev. Lett. 1999, 83, 4840.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(40) Nahum, E.; Ebenstein, Y.; Aharoni, A.; Mokari, T.; Banin, U.; Shimoni, N.; Millo, O. Transport and Charging in Single Semiconductor Nanocrystals Studied by Conductance Atomic Force Microscopy. Nano Lett. 2004, 4, 103108.

Page 14 of 16

of Luminescence in Colloidal CdSe/ZnS Quantum Dots. Nanotechnol. 2005, 16, 1517. (48) Wang, X.; Qu, L.; Zhang, J.; Peng, X.; Xiao, M. Surface-Related Emission in Highly Luminescent CdSe Quantum Dots. Nano Lett. 2003, 3, 11031106.

(41) Jiang, Y.; Qi, Q.; Wang, R.; Zhang, J.; Xue, Q.; Wang, C.; Jiang, C.; Qiu, X. Direct Observation and Measurement of Mobile Charge Carriers in a Monolayer Organic Semiconductor on a Dielectric Substrate. ACS Nano 2011, 5, 61956201.

(49) Song, M.; Wu, B.; Chen, G.; Liu, Y.; Ci, X.; Wu, E.; Zeng, H. Photoluminescence Plasmonic Enhancement of Single Quantum Dots Coupled to Gold Microplates. J. Phys. Chem. C 2014, 118, 85148520.

(42) Miyajima, S.; Irikawa, J.; Yamada, A.; Konagai, M. High Quality Aluminum Oxide Passivation Layer for Crystalline Silicon Solar Cells Deposited by ParallelPlate Plasma-Enhanced Chemical Vapor Deposition. Appl. Phys. Express 2009, 3, 012301.

(50) Kulakovich, O.; Strekal, N.; Yaroshevich, A.; Maskevich, S.; Gaponenko, S.; Nabiev, I.; Woggon, U.; Artemyev, M. Enhanced Luminescence of CdSe Quantum Dots on Gold Colloids. Nano Lett. 2002, 2, 14491452.

(43) Li, T.-T.; Cuevas, A. Eective Surface Passivation of Crystalline Silicon by RF Sputtered Aluminum Oxide. Phys. Status Solidi Rapid Res. Lett. 2009, 3, 160162.

(51) Efros, A. L. Nanocrystals: Almost Always Bright. Nat. Mater. 2008, 7, 612613. (52) Tvrdy, K.; Frantsuzov, P. A.; Kamat, P. V. Photoinduced Electron Transfer from Semiconductor Quantum Dots to Metal Oxide Nanoparticles. Proc. National Acad. Sci. 2011, 108, 2934.

(44) König, T.; Simon, G. H.; Heinke, L.; Lichtenstein, L.; Heyde, M. Defects in Oxide Surfaces Studied by Atomic Force and Scanning Tunneling Microscopy. Beilstein J. Nanotechnol. 2011, 2, 114.

(53) Mukherjee, S.; Maiti, R.; Katiyar, A. K.; Das, S.; Ray, S. K. Novel Colloidal MoS2 Quantum Dot Heterojunctions on Silicon Platforms for Multifunctional Optoelectronic Devices. Sci. Rep. 2016, 6 .

(45) Serebrennikova, I.; White, H. S. Scanning Electrochemical Microscopy of Electroactive Defect Sites in the Native Oxide Film on Aluminum. Electrochem. SolidState Lett. 2001, 4, B4B6.

(54) Liu, H.-Y.; Xu, B.; Wei, Y.-Q.; Ding, D.; Qian, J.-J.; Han, Q.; Liang, J.-B.; Wang, Z.-G. High-Power and LongLifetime InAs/GaAs Quantum-Dot Laser at 1080 nm. Appl. Phys. Lett. 2001, 79, 28682870.

(46) Sullivan, J.; Barbour, J.; Dunn, R.; Son, K.; Montes, L.; Missert, N.; Copeland, R. The Electrical Properties of Native and Deposited Thin Aluminum Oxide Layers on Aluminum: Hydration Eects. Proc. 3rd Symp. on Critical Factors in Localized Corrosion, Electrochem. Soc. 1999; pp 1117.

(55) Akiyama, T.; Kuwatsuka, H.; Simoyama, T.; Nakata, Y.; Mukai, K.; Sugawara, M.; Wada, O.; Ishikawa, H. Nonlinear Gain Dynamics in QuantumDot Optical Ampliers and its Application to Optical Communication Devices.

(47) Lee, W.; Shu, G.; Wang, J.; Shen, J.; Lin, C.; Chang, W.; Ruaan, R.; Chou, W.; Lu, C.; Lee, Y. Recombination Dynamics


14

Page 15 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry IEEE

J.

Quantum

10591065.

Electron.

2001,

37

,

(56) Luther, J. M.; Law, M.; Beard, M. C.; Song, Q.; Reese, M. O.; Ellingson, R. J.; Nozik, A. J. Schottky Solar Cells Based on Colloidal Nanocrystal Films. Nano Lett. 2008, 8, 34883492.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Graphical TOC Entry


16

Page 16 of 16

Ultrafast Emission Decay and Enhancement of ... - ACS Publications

Recommend Documents