Demystifying Complex Quantum Dot Heterostructures Using

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Demystifying Complex Quantum Dot Heterostructures Using Photo-Generated Charge Carriers G. Krishnamurthy Grandhi, and Ranjani Viswanatha J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 21 Apr 2017 Downloaded from http://pubs.acs.org on April 23, 2017

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The Journal of Physical Chemistry Letters

Demystifying Complex Quantum Dot Heterostructures Using Photo-generated Charge Carriers G Krishnamurthy Grandhi[a] and Ranjani Viswanatha*[a,b] [a]

New Chemistry Unit and [b]International Centre for Materials Science, Jawaharlal Nehru

Centre for Advanced Scientific Research, Jakkur, Bangalore-560064, India. AUTHOR INFORMATION Corresponding Author * [email protected]

ABSTRACT. The success of heterostructure quantum dots in optoelectronic and photovoltaic applications is based on our understanding of photo-generated charge carrier localization. However, often the actual location of charge carriers in heterostructure semiconductors is quite different from their predicted positions leading to sub-optimal results. In this work, photoluminescence of Cu doped heterostructures have been used to study the charge localization of alloys, inverse type I, type II and quasi type II core/shell structures and graded alloys. Specifically, the adeptness of this method has been assessed over a range of widely studied heterostructures like CdSe/CdS, CdS/CdSe, CdSe/CdTe, Zn1-xCdxSe and Zn1-xCdxS quantum

1 ACS Paragon Plus Environment


dots systems by doping them with a small percentage of Cu. The electron and hole localization obtained from this method concurs with the pre-existing understanding in cases that have been explored before while the internal structure of previously unknown heterostructures have been predicted.

Cu PL

Cu PL

BE PL

BE PL

TOC GRAPHICS:

Cu PL

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

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Cu level

KEYWORDS. electronic structure, shell thickness, electron and hole energies, delocalization and Cu doping. Quantum dot (QD) heterostructures have been an integral part of the development of nano devices for use in optoelectronic applications. High PL efficiencies suitable for light emitting diodes (LEDs),1 and photo-voltaic materials suitable for solar cell applications2,

3

have been

attained not only by appropriate alignment of band edges but also due to a multitude of other factors like smooth alloying to reduce Auger recombination,4 appropriate surface passivation5 and so on. Additionally, due to high temperature annealing during synthesis and the strains involved in the formation of these heterostructures the theoretically planned structure is rarely formed. The challenges involved in realizing a high-quality material to maximize the efficiency of either the charge separation or recombination processes can be two-fold. Firstly, the band offset values have been obtained largely in bulk semiconductors. The electronic structure


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prediction of these heterostructures is not straight forward when it comes to nano regime as their band offset values may not be similar to bulk values. In addition to a few theoretical reports,6-8 experimental techniques such as variable energy photoemission (VUV),9 scanning tunneling spectroscopy (STS)10 and cyclic voltammetry (CV)

11, 12

were used to study the electronic

structure of a few QD heterostructures. However, these measurements require ultrahigh vacuum, especially VUV and STS, rendering the measurements non-trivial and time consuming. Moreover, charging of the semiconductor QDs11 in all these techniques poses an important shortcoming in terms of determining the shift in conduction band (CB) or valence band (VB) positions. Ultrafast transient absorption spectroscopy was also utilized to find the type of band alignment in CdTe/CdSe13 and CuInS2/CdS14 core-shell QDs. However, the absolute positions of these QDs were not determined using this technique. Secondly, due to the quantum confinement in these materials, several other factors like defect and strain energies also influence the localization effects on the photo-generated charge carrier. Hence an in-situ probe to study the internal structure would be very useful in obtaining high quality heterostructure suitable for the given application. A precise in-situ probe identifying the relative energies of CB and VB using the location of the charge carrier in various Cu doped II-VI and III-V semiconductors has been shown to effectively predict band edge variations.15 16 Dilute Cu doping introduces a single atomic-like energy level in between the CB and VB of the host material. The atomic-like nature of this level shields it from the energy variations arising due the band broadening, strain within the lattice and other solid state effects. Hence the energy of the radiative recombination arising from the CB of the host to this atomic-like state is a measure of the shift in the CB energy levels.17 The Cu level energy for the various host materials is found to be constant within ±25 meV for the II-VI


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semiconductors.15 This method was shown to be in good agreement with both theoretical and other experimental results. Further, it is important to note that it is necessary to track changes in the energy level with respect to any standard energy giving us one degree of freedom. Using this degree of freedom, we have set the Cu energy level aligned with the bulk CdS energy levels. The importance and novelty of this method in finding the relative band edge positions is also highlighted in a recently published review on Cu doped QDs.18 The advantages of using this method include the ease of operation under atmospheric conditions using simple optical measurements like absorption and photoluminescence. The presence of internal standard, similar to this atomic-like level of Cu allows the direct determination of photo-generated electron (PGE) energy from the Cu PL maximum and the photo generated hole (PGH) energy can be obtained by subtracting the band gap energy from Cu PL maximum. The variation in Cu level from system to system has been included as the error bars for PGE and PGH values through the manuscript. Additionally, due to the kinetics of this recombination, it was shown that the Cu emission is enhanced in absence of photo-generated hole in the VB. Hence, the ratio of Cu emission to the band edge emission is a measure of extent of hole trapping on the host QD surface.15, 17,19 However, the major bottleneck for the use of this method arises from a synthesis perspective. Although Cu doping in II-VI and III-V semiconductors has been extensively studied, there are not many reports on Cu doping in QD heterostructures. This could be due to the need to maintain lower temperatures to retain Cu within the QD20 while high temperature annealing for extended period of time is necessary for the formation of high quality heterostructures.21 In this manuscript, we have synthesized Cu doped heterostructures by the appropriate use of ligands as well as suitable variation of temperatures. We have then shown that these Cu doped heterostructures can be used to


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understand the charge localization in a range of heterostructure QDs including type I, type II, inverse type I, quasi type II QDs along with alloys and alloy shell heterostructures using Cu emission and absorption spectra. The validity of this measurement over a wide range of materials proves the generality of this approach. One of the most widely studied heterostructures in literature is the CdSe/CdS core/shell QDs with a thin as well as thick shell.1, 21, 22 Theoretically, the structure of CdSe/CdS is expected to be type I structure from the bulk band edges wherein both the charge carriers are localized in CdSe core. However, since the earliest reports of the formation of these QD heterostructures,22 along with an increase in PL quantum yield, we also observe a decrease in the band gap as a function of shell thickness which is unexpected in ideal type I heterostructures similar to CdSe/ZnS.5, 23 Charge carrier delocalization beyond the traditional boundaries of the core and/or shell is well known in literature due to change in material composition as well as energetics. In the current case, this red shift was observed in many literature reports21, 22 which predict a quasi type II nature of CdSe/CdS QDs wherein the electron is delocalized into CdS shell as a result of small CB offset. However, this quasi type II character does not completely explain the observed optical behavior. In order to predict the optical behavior in heterostructures, it is crucial to understand the internal structure of these QDs. To study the internal structure, we prepared Cu doped CdSe/CdS heterostructure QDs. The formation of CdS shell over CdSe core is confirmed from X-ray diffraction (XRD) as well as the growth observed from transmission electron microscope (TEM) images. It is well known that it is not possible to differentiate the presence of a core and shell material even from high resolution TEM (HRTEM) images. Hence we studied the optical properties of both undoped and Cu doped QDs and typical absorption and PL spectra for the undoped and doped QDs for varying shell thicknesses are shown in Figure 1a and Figure 1b



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respectively. The red shifted emission and the long lifetime of this emission (shown in Figure S1) confirms the origin of the emission peak in Cu doped QDs as due to radiative recombination of CB electron with Cu d-level.17 The surface ligand chemistry15, 17 is tuned to obtain both the band edge and Cu emission as shown in Figure 1b to verify the corresponding Stokes’ shift in the system. This Cu PL energy is indicative of the energy of the PGE while the difference between band gap and the Cu PL energy is indicative of the PGH energy. In order to avoid confusion regarding the VB and CB of the core and/or shell, in the entire manuscript we discuss the energy of PGE and PGH rather than the conduction band and valence band. The variation of energy of the PGE (red open circles) and the PGH (black open circles) of CdSe/CdS with increasing CdS shell thickness are compiled in Figure 1c. The dotted lines in Figure 1c show the variation of CB (red dashed line) and VB (black dashed line) of CdSe QDs within the same size regime. The decrease in energy of the PGE is very similar to the variation of the CB of CdSe for thinner shells of CdS suggesting that the electron experiences an increase in overall size of QD with almost similar potential to that of CdSe with increasing CdS shell thickness. This, in turn, implies a delocalization of the PGE into the CdS shell due to small CB offset in CdSe and CdS QDs supporting the quasi type II nature of CdSe/CdS predicted in literature.6,

22

However,

surprisingly, two aspects that cannot be explained using the quasi type II nature is the relative flattening of the PGE energy at thicker shells of CdS as well as small increase in the energy of the PGH for thinner shells. For thin shells, the variation of PGE energy is similar to the CdSe size variation suggesting largely CdSe type wavefunction. However, with increasing shell thickness, the wave-function distinctly turns to CdS type character with the variation similar to that of the CdS QDs. It is well known that the excitonic bohr radius of CdS (~3nm) is much smaller than that of CdSe (~5 nm). Hence, the variation of CdS would decrease much more


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sharply than that of CdSe as shown in Figure 1c. So, with increase in CdS character, we observe that the energy of the electron is similar to that of CdS. In this context, it is interesting to note that extended annealing in this CdSe/CdS structure can give rise to a graded alloy at the interface. The thickness of this alloy at the interface from these studies can be found to be about one monolayer thick. Similar alloy interface was also predicted through indirect evidence of decrease in Auger- recombination rates in CdSe/CdS QDs.24 The presence of a graded alloy at the interface would gradually blend the band edges of CdSe and CdS. Due to smaller energy difference between the CB edges of CdSe and CdS, the PGE would delocalize into the entire region while the PGH delocalizes into a much smaller shell region, possibly only into the alloy region at the interface. Hence we observe a small increase in the energy levels of PGH and larger decrease in the PGE energies. This is also in accordance with literature4 wherein the presence of a deliberately manicured sharp interface between CdSe/CdS shows a smaller red shift in the PL (~ 0.27 eV) compared to a graded alloy interface (up to ~ 0.34 eV) of same total size, proving the power of this technique.



(a)

CdS shell thickness

CdSe/CdS

(c)

-3.1

Core size=2.8 nm

Undoped CdSe/CdS

-3.3

(b)

CdS shell thickness

Cu doped CdSe/CdS

Norm. Intensity

2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0

-3.4 0.14 eV CdSe

CdS

PGE (CdSe/CdS) PGH (CdSe/CdS) CB (CdSe) VB (CdSe) CB (CdS)

Cu level

-5.8

0.41 eV

Energy Vs vacuum (eV)

-3.2

CdSe core

Norm. Absorbance

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-5.9

CdSe core

-6.0

2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0

Energy (eV)

2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8

Size (nm)

Figure 1. (a) and (b) show the normalized absorption and PL spectra of undoped and Cu doped CdSe/CdS QDs respectively. λex=405 nm. (c) The variation of energies of PGE (red open circles) and PGH (black open circles) of CdSe/CdS QDs with increasing the CdS shell thickness. The solid lines are a guide to the eye. The dashed lines indicate the variation of CB (red),VB (black) of CdSe QDs and CB (blue) of CdS QDs which are obtained from literature.15 The inset of (c) shows the bulk band alignment of CdSe/CdS25 along with the relative position of Cu level.15 We have then extended this technique to study the internal structure of the inverse type I CdS/CdSe structure. The overcoating of CdSe on Cu doped CdS QDs leads to red shift in both band gap and Cu PL as shown in Figure 2a and Figure 2b respectively. The variations of PGE and PGH energies are plotted in Figure 2c while the bulk band offset is shown in the corresponding inset. The PGE energy is shifted by 210 meV when a 0.25 nm thick CdSe shell is overcoated on 3.0 nm CdS QDs, while the PGH energy is shifted by 670 meV. These shifts are


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comparable to the corresponding bulk CB offset of 140 meV and VB offset of 410 meV. Additionally, the PGH energy becomes constant at higher shell thickness suggesting the localization of both PGH and PGE within CdSe shell with increasing shell thickness. The efficiency of the hole transfer to the shell is slightly higher than that of the electron due to the large VB offset of the component materials. Thus, CdS/CdSe QDs form inverse type I structure

CdS/CdSe (c) Core size= 3.0 nm

(a)

CdSe shell thickness

-3.0 -3.2

0.14 eV CdS

Cu CdS

-3.4

CdSe

Cu level

-3.6

PGE Energy PGH Energy

3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6

0.41 eV

(b)

-5.8 -6.0 CdSe shell Cu CdS thickness


Norm. Absorbance

even in quantum confined regime similar to bulk alignment.

Norm. Intensity

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


-6.2 -6.4

3.0

2.5

2.0

Energy (eV)

1.5

1.0

3.0

3.1

3.2

3.3

Size (nm)

3.4

3.5

Figure 2. (a) and (b) are the normalized absorption and PL spectra of Cu doped CdS/CdSe QDs. λex=405 nm. (c) The variation of energies of PGE (red open circles) and PGH (black open circles) of CdS/CdSe QDs with increasing the CdSe shell thickness. The solid line is a guide to the eye. The inset of (c) shows the bulk band alignment of CdS/CdSe25 along with the relative position of Cu level.15



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The versatility of this technique was explored by extending our study to type II CdSe/CdTe QDs semiconductor. We have first studied Cu doped CdSe/CdTe QDs with 3.4 nm CdSe core. The absorption and PL spectra of these QDs with increasing shell thickness are shown in Figure S2. Figure 3a shows the variation of PGE and PGH as a function of shell thickness. From the figure, it is evident that the PGE energy is almost constant with increasing CdTe shell thickness while the PGH energy increases with increasing shell thickness consistent with the expected type II nature of the interface. Specifically, we observe the localization of electron within core while the hole is getting localized into the shell with increasing shell thickness. Theoretically, it was shown that these type II QDs show quasi type II nature for a thin shell on top of a small core (diameter < 4 nm).8 However, it is well known that though these effective mass based theoretical calculations are known to be qualitatively correct, quantitative values may not be experimentally correct. Hence though the recommended core diameter is lower than 4 nm, we only observe type II alignment for CdSe/CdTe QDs even with a 3.4 nm CdSe core. However, we have studied the effect of core diameter on the electronic structure of type II QDs by synthesizing CdSe/CdTe QDs with a smaller diameter of (2.5 nm) CdSe core. Their absorption and PL spectra are shown in Figure S3. Figure 3b shows the variation of PGE and PGH energies as a function of shell thickness. Interestingly, the PGE energy decreases with increasing CdTe thickness until the overall diameter of core-shell QDs reaches approximately 5 nm and thereafter remains constant. The variation of PGH energy is similar to the earlier case and is monotonously increasing with increasing shell thickness. The continuous increase in PGH energy with increasing CdTe shell thickness is a consequence of hole localization into CdTe shell. The initial decrease in electron energy indicates the electron delocalization into CdTe shell which suggests that smaller cores


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with thinner shells are consistent with a quasi type II structure. Further increase in shell thickness leads to a regular type II structure.

(a)

-3.2

(b) Quasi type-II/type-II CdSe/CdTe

Type-II CdSe/CdTe Core size= 3.4 nm

Core size= 2.5 nm

-3.0 -3.1 -3.2

-3.3 -3.4

-3.3

0.30 eV CdSe

-3.5

CdTe

-3.4 -3.5

Cu level

-3.6 -5.4



0.61 eV

-5.5

-5.6 -5.7

-5.6

-5.8

-5.7 -5.8

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-6.1 10

Size (nm)

Figure 3. The variation of energies of PGE (red open circles) and PGH (black open circles) of CdSe/CdTe QDs with (a) 3.4 nm CdSe core and (b) 2.5 nm CdSe core with increasing shell thickness. The solid line is a guide to the eye. The inset of (a) shows the bulk band alignment of CdSe/CdTe25 along with the relative position of Cu level.15 To further explore the widespread applicability of this technique, we have studied the internal structure of alloy QDs. Zn1-xCdxSe characterized from TEM shows no change in size while the XRD shows a shift in the peaks from bulk ZnSe to bulk CdSe like structure with the incorporation of Cd confirming the formation of alloys as shown in Figure S4. Both the band gap and PL energy of undoped QDs decrease with increase in Cd composition in the alloy QDs



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(Figure S5a). The corresponding Cu doped counter parts show the dominant Cu emission in the PL spectra as shown in Figure S5b. Figure 4a shows the variation in PGE and PGH energies with increase in Cd composition in the QDs. The band gap variation is dominated by decrease in the PGE energy with increase in Cd composition whereas PGH energy is largely constant. This can be understood as a result of higher CB offset compared to the VB offset in ZnSe/CdSe system in bulk as shown in the inset of Figure 4a. Similar studies on the internal structure of Zn1-xCdxS alloy QDs for two different sizes are shown in Figure 4b. The XRD characterization shows the formation of Zn1-xCdxS alloy QDs as shown in Figure S6a. The TEM also supports the alloy formation and the sizes of QDs were found to be 4.2 nm and 4.6 nm respectively as seen from Figure S6b and Figure S6c. The absorption and PL spectra of undoped and Cu doped Zn1-xCdxS alloy QDs are shown in Figure S7. Similar to the case of Zn1-xCdxSe alloy QDs, the PGE energy decreases with increase in Cd composition of Zn1-xCdxS alloy QDs whereas the PGH energy remains largely constant. This variation is a consequence of bulk band alignment of ZnS/CdS (inset of Figure 4b). Further with a decrease in size of ZnS core, we observe a smaller shift in the PGH energy compared to the PGE energy consistent with the variation of band edges as a function of size in ZnS QDs.


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(a)

Zn0.98-XCdXCu0.02Se

-3.0

(b)

Zn0.99-XCdXCu0.01S

-2.0

Core size= 6.6 nm

-2.5 -3.5



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-3.0 -4.0

-3.5 PGE energy PGH energy

-5.0

0.73 eV ZnSe

CdSe

ZnS

CdS

Cu level

Cu level

-5.5

-5.5

PGE Energy (2.8 nm) PGH Energy (2.8 nm) PGE Energy (4.2 nm) PGH Energy (4.2 nm) -6.0

1.02 eV

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0.23 eV

-6.5

-6.0 0

20

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0

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% Cd

% Cd

Figure 4. (a) The variations of energies of PGE (red open circles) and PGH (black open circles) of Zn0.98-xCdxCu0.02Se QDs with increasing Cd composition. (b) The variations of energies of PGE (red circles) and PGH (black circles) of Zn0.98-xCdxCu0.01S QDs of 2.8 nm (closed circles) and 4.2 nm (open circles) ZnS core QDs with increasing Cd composition. The solid lines in every case is a guide to the eye. The insets of (a) and (b) show the bulk band alignments of ZnSe/CdSe and ZnS/CdS respectively25 along with the relative position of Cu level.15 Further, this method was used to study the internal structure variation due to the variation of the precursor reactivity exploiting the well-known difference in reactivity of the Cd precursor compared to that of Zn. We modified the above synthetic conditions by swiftly adding all Cd2+ precursor in presence of excess Zn2+ in the reaction flask and annealing the reaction mixture for similar period of time as in previous case. In this case, TEM images obtained by slow and swift addition of Cd are shown in Figure 5a and 5b respectively showing an increase from 4.2 nm to



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5.7 nm in the size of QDs. However, surprisingly, the bigger 5.7 nm ZnS/CdS QDs (closed circles) show higher band gap than the smaller 4.2 nm Zn1-xCdxS alloy QDs (open circles) for a given Cd composition as shown in Figure 5c. In order to understand the nature of these bigger QDs, we studied the PL quantum yield as shown in Figure 5d. These bigger QDs showed an enhanced Cu PL compared to the smaller QDs of similar band gap suggesting a formation of a ZnS shell around the alloy of ZnxCd1-xS QDs with a type I alignment. This was further confirmed by controlled etching of the surface of these QDs as shown in Figure S8 demonstrating a decrease in the Zn composition from 33% to 15% subsequent to the etching reinforcing the ZnS rich shell formation. However, the thickness of the shell is not evident from these experiments due to their type I alignment. Simple analysis of the band gap variation in the case of alloy/shell material suggests that the percentage of Cd within the alloy/ZnS shell would be higher due to the presence of a ZnS shell that is not involved in the emission process but contributes Zn to the total percentage. So, it is reasonable to expect lower band gap for QDs than the alloy only QDs. However, we have observed higher band gap for alloy/shell QDs than alloy only QDs for a given Cd/Zn. This is only possible if the size of alloy core in alloy/ZnS QDs is smaller than size of alloy only QDs as the smaller size particles exhibit higher band gap (as seen from Figure 4b). However, while we can figure out that the size of the alloy core is smaller than 4.2 nm, the actual size of the core cannot be determined from this method. The comparison of the variations of PGE and PGH energies of both alloy QDs and alloy/ZnS core-shell QDs as a function of Cd composition is shown in Figure 5e. The PGE energy variation remains similar in both cases while the PGH energy decreases faster in alloy-shell QDs compared to the alloy only QDs.


(b)

20 20nm nm

Norm. Absorbance

(a)

20 nm nm 20

2.0

(d) 2.0

Smaller QDs Bigger QDs

1.5

1.5

1.0

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0.0 0.0 4.0 3.5 3.0 2.5 2.0 1.5 1.0 (e)

(c)

Smaller QDs Bigger QDs

3.8

Core size=4.2 nm

3.6

Core size=4.2 nm

-2.5 -3.0

3.4 -3.5

3.2 PGE Energy PGH Energy

Smaller QDs Larger QDs

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Norm. Intensity

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-7.0

Figure 5. TEM images of the (a) smaller 4.2 nm QDs and (b) the bigger 5.7 nm QDs obtained by swift addition of Cd2+ precursor. (c) The band gap variations of smaller QDs and bigger QDs with varying Cd composition. (d) The absorption and PL spectra of smaller and bigger Cu doped alloy QDs. (e) The variation of energies of PGE (red circles) and PGH (black circles) of Zn0.98xCdxCu0.01S

alloy (open circles) and alloy/ZnS core-shell (closed circles) QDs as a function of

Cd composition. The solid lines are a guide to the eye. In conclusion, we have studied the internal structure of CdSe/CdS, CdS/CdSe, CdSe/CdTe, Zn1-xCdxSe and Zn1-xCdxS QDs by doping Cu. In case of CdSe/CdS QDs, the PGE delocalizes over the entire QD while the PGH delocalization is restricted to the alloy interface. The CdTe overcoating on CdSe core QDs of larger size results in formation of type II CdSe/CdTe QDs whereas it leads to the formation of quasi type II structure for smaller core sizes qualitatively



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consistent with the theoretical predictions. We have also shown that this method can be useful in studying the internal structure of previously unknown heterostructures like Zn1-xCdxS/ZnS alloyshell QDs. Thus, having successfully predicted the internal structure of a wide range of heterostructures, this method can be widely applied to study heterostructure interfaces and hence correlate their properties to the corresponding structures. EXPERIMENTAL SECTION. Synthesis of heterostructures QDs. Copper stearate (CuSt2) was synthesized and purified similar to literature reported procedure.20 Cu doped CdSe/CdS QDs were synthesized by first synthesizing Cu doped CdSe QDs using similar to literature reported method15 and purified by washing once with hexane-methanol mixture and then precipitated by adding excess of acetone. The over coating of CdS shell was achieved by successive ionic layer and adsorption reaction (SILAR) technique.21, 22 The undoped CdSe/CdS QDs were synthesized exactly in the same way excluding the Cu addition step. Similarly, Cu doped CdS/CdSe QDs were synthesized by first synthesizing Cu doped CdS QDs using similar to literature reported method.26 After this, CdSe over coating was achieved by SILAR technique.27 Cu doped CdSe QDs were prepared similar to the mentioned method above at temperatures of 180 oC and 250 oC respectively for the 2.5 nm and 3.4 nm CdSe QDs. SILAR technique was used for overcoating of CdTe shell.3 Cu doped Zn1-xCdxSe alloy QDs were synthesized similar to literature reported methods.17 Cu doped alloy Zn1-xCdxS QDs were obtained by slow addition of cadmium oleate to the Cu doped ZnS QDs in presence of excess Zn2+. On the other hand, Cu doped Zn1-xCdxS/ZnS shell QDs were synthesized by swift addition of cadmium oleate into a solution of Cu doped ZnS QDs and excess Zn2+ in one shot and annealed for same period of time as in case of Cu doped alloy Zn1-xCdxS QDs. The etching of


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Zn1-xCdxS QDs was achieved by sonicating a mixture of 1-2 drops of H2O2 in 3 ml hexane solution of QDs (O.D. ~3 at absorption maximum) and 0.2 ml of oleic acid for 5-10 minutes till the absorption maximum gets blue shifted. The detailed synthesis procedures are discussed in the supporting information. Characterization and spectroscopic studies. Crystal structure identification of the particles was carried out using XRD, recorded on Bruker D8 Advance Diffractometer using Cu-Kα radiation. All patterns were recorded at a slow scan rate (0.75o per minute) in order to get a high signal-to noise ratio. The bulk XRD was obtained from the inorganic crystal structure database. Relevant XRD patterns for various semiconductor QDs are shown in Figure S9, supporting information. TEM images were recorded using Technai F30UHR version electron microscope, using a field emission gun (FEG) at an accelerating voltage of 200 kV. Samples for TEM were washed in chloroform-acetone mixture. A drop of purified QDs which were dissolved in hexane was put on carbon coated Cu grid. The solution was allowed to evaporate leaving behind the QDs. Representative TEM images for various semiconductor QDs are shown in Figure S10. UV-visible absorption spectra of various aliquots dissolved in hexane and chloroform were obtained using Agilent 8453 UV-visible spectrometer. The minimum of the first derivative of the absorption is taken as the band gap of the material. Steady state PL spectra were collected using the 450 W xenon lamp as the source on the FLSP920 spectrometer, Edinburgh Instruments, while the lifetime measurements were carried out using EPL-405 pulsed diode laser as an excitation source (λex = 405 nm). Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) was used to determine the Cu to Cd ratio in the doped QDs. QDs were washed to remove excess precursors and then digested in concentrated HNO3 which were further diluted with Millipore water. ICP-OES was carried out



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using Perkin Elmer Optima 7000 DV Machine. ~1% of Cu doping was found in all the Cu doped samples. The ICP-OES data on the etched samples shows that Cu dopants are mostly uniformly distributed in these heterostructures samples as shown in Table S1. The energies of PGE and PGH of all the core-shell QDs and alloy QDs that were discussed in this manuscript are shown with respect to vacuum scale by matching the energies of PGE and PGH of corresponding core to the literature values.15 ASSOCIATED CONTENT Supporting Information. The detailed synthesis procedures of various heterostructures QDs and their XRD patterns, TEM images, absorption spectra and PL spectra are given. AUTHOR INFORMATION Notes The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interests. ACKNOWLEDGMENT We thank JNCASR, Sheikh Saqr Laboratory, and the Department of Science and Technology, Government of India for financial support. G.K.G. thanks CSIR for a research Fellowship. R.V. is grateful for the Sheikh Saqr Career award. REFERENCES (1)

Pal, B. N.; Ghosh, Y.; Brovelli, S.; Laocharoensuk, R.; Klimov, V. I.; Hollingsworth, J.

A.; Htoon, H. ‘Giant’ CdSe/CdS Core/Shell Nanocrystal Quantum Dots As Efficient


Page 19 of 22 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


Electroluminescent Materials: Strong Influence of Shell Thickness on Light-Emitting Diode Performance. Nano Lett. 2012, 12, 331-336. (2)

Pan, Z.; Zhang, H.; Cheng, K.; Hou, Y.; Hua, J.; Zhong, X. Highly Efficient Inverted

Type-I CdS/CdSe Core/Shell Structure QD-Sensitized Solar Cells. ACS Nano 2012, 6, 39823991. (3)

McElroy, N.; Page, R. C.; Espinbarro-Valazquez, D.; Lewis, E.; Haigh, S.; O'Brien, P.;

Binks, D. J. Comparison of Solar Cells Sensitised by CdTe/CdSe and CdSe/CdTe Core/Shell Colloidal Quantum Dots with and without a CdS Outer Layer. Thin Solid Films 2014, 560, 6570. (4)

Bae, W. K.; Padilha, L. A.; Park, Y.-S.; McDaniel, H.; Robel, I.; Pietryga, J. M.; Klimov,

V. I. Controlled Alloying of the Core-Shell Interface in CdSe/CdS Quantum Dots for Suppression of Auger Recombination. ACS Nano 2013, 7, 3411-3419. (5)

Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Highly

Luminescent

Monodisperse

CdSe

and

CdSe/ZnS

Nanocrystals

Synthesized

in

a

Hexadecylamine-Trioctylphosphine oxide-Trioctylphospine Mixture. Nano Lett. 2001, 1, 207211. (6)

Li, J.; Wang, L.-W. First Principle Study of Core/Shell Structure Quantum Dots. Appl.

Phys. Lett. 2004, 84, 3648-3650. (7)

Pokrant, S.; Whaley, K. B. Tight-Binding Studies of Surface Effects on Electronic

Structure of CdSe Nanocrystals: the Role of Organic Ligands, Surface Reconstruction, and Inorganic Capping Shells. Eur. Phys. J. D 1999, 6, 255-267.



(8)

Page 20 of 22

Piryatinski, A.; Ivanov, S. A.; Tretiak, S.; Klimov, V. I. Effect of Quantum and Dielectric

Confinement on the Exciton-Exciton Interaction Energy in Type II Core/Shell Semiconductor Nanocrystals. Nano Lett. 2007, 7, 108-115. (9)

Santra, P. K.; Viswanatha, R.; Daniels, S. M.; Pickett, N. L.; Smith, J. M.; O'Brien, P.;

Sarma, D. D. Investigation of the Internal Heterostructure of Highly Luminescent Quantum Dot− Quantum Well Nanocrystals. J. Am. Chem. Soc. 2008, 131, 470-477. (10) Steiner, D.; Dorfs, D.; Banin, U.; Della Sala, F.; Manna, L.; Millo, O. Determination of Band Offsets in Heterostructured Colloidal Nanorods Using Scanning Tunneling Spectroscopy. Nano Lett. 2008, 8, 2954-2958. (11) Li, Y. C.; Zhong, H. Z.; Li, R.; Zhou, Y.; Yang, C. H.; Li, Y. F. High-Yield Fabrication and Electrochemical Characterization of Tetrapodal CdSe, CdTe, and CdSexTe1-x Nanocrystals. Adv. Funct. Mater. 2006, 16, 1705-1716. (12) Hou, B.; Parker, D.; Kissling, G. P.; Jones, J. A.; Cherns, D.; Fermín, D. J. Structure and Band Edge Energy of Highly Luminescent CdSe1–xTex Alloyed Quantum Dots. J. Phys. Chem. C 2013, 117, 6814-6820. (13) Chuang, C.-H.; Lo, S. S.; Scholes, G. D.; Burda, C. Charge Separation and Recombination in CdTe/CdSe Core/Shell Nanocrystals as a Function of Shell Coverage: Probing the Onset of the Quasi Type-II Regime. J. Phys. Chem. Lett. 2010, 1, 2530-2535. (14) Wu, K.; Liang, G.; Kong, D.; Chen, J.; Chen, Z.; Shan, X.; McBride, J. R.; Lian, T. Quasi-Type II CuInS2/CdS Core/Shell Quantum Dots. Chem. Sci. 2016, 7, 1238-1244.


Page 21 of 22 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


(15) Grandhi, G. K.; Tomar, R.; Viswanatha, R. Study of Surface and Bulk Electronic Structure of II–VI Semiconductor Nanocrystals Using Cu as a Nanosensor. ACS Nano 2012, 6, 9751-9763. (16)

Zhang, Z.; Li, D.; Xie, R.; Yang, W. Insights into the Energy Levels of Semiconductor

Nanocrystals by a Dopant Approach. Angew. Chem. Int. Ed. 2013, 52, 5052-5055. (17) Viswanatha, R.; Brovelli, S.; Pandey, A.; Crooker, S. A.; Klimov, V. I. Copper-Doped Inverted Core/Shell Nanocrystals with “Permanent” Optically Active Holes. Nano Lett. 2011, 11, 4753-4758. (18) Knowles, K. E.; Hartstein, K. H.; Kilburn, T. B.; Marchioro, A.; Nelson, H. D.; Whitham, P. J.; Gamelin, D. R. Luminescent Colloidal Semiconductor Nanocrystals Containing Copper: Synthesis, Photophysics, and Applications. Chem. Rev. 2016, 116, 10820–10851. (19) Grandhi, G. K.; M, A.; Viswanatha, R. Understanding the Role of Surface Capping Ligands in Passivating the Quantum Dots Using Copper Dopants as Internal Sensor. J. Phys. Chem. C 2016, 120, 19785-19795. (20) Chen, D.; Viswanatha, R.; Ong, G. L.; Xie, R.; Balasubramaninan, M.; Peng, X. Temperature Dependence of “Elementary Processes” in Doping Semiconductor Nanocrystals. J. Am. Chem. Soc. 2009, 131, 9333-9339. (21) Saha, A.; Chellappan, K. V.; Narayan, K. S.; Ghatak, J.; Datta, R.; Viswanatha, R. NearUnity Quantum Yield in Semiconducting Nanostructures: Structural Understanding Leading to Energy Efficient Applications. J. Phys. Chem. Lett. 2013, 4, 3544-3549.



Page 22 of 22

(22) Li, J. J.; Wang, Y. A.; Guo, W.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. Large-Scale Synthesis of Nearly Monodisperse CdSe/CdS Core/Shell Nanocrystals Using AirStable Reagents via Successive Ion Layer Adsorption and Reaction. J. Am. Chem. Soc. 2003, 125, 12567-12575. (23) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. (CdSe)ZnS Core−Shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites. J. Phys. Chem. B 1997, 101, 9463-9475. (24) García-Santamaría, F.; Brovelli, S.; Viswanatha, R.; Hollingsworth, J. A.; Htoon, H.; Crooker, S. A.; Klimov, V. I. Breakdown of Volume Scaling in Auger Recombination in CdSe/CdS Heteronanocrystals: The Role of the Core-Shell Interface. Nano Lett. 2011, 11, 687693. (25) Norris, D. J.; Efros, A. L.; Erwin, S. C. Doped Nanocrystals. Science 2008, 319, 17761779. (26) Grandhi, G. K.; Viswanatha, R. Tunable Infrared Phosphors Using Cu Doping in Semiconductor Nanocrystals: Surface Electronic Structure Evaluation. J. Phys. Chem. Lett. 2013, 4, 409-415. (27) Battaglia, D.; Li, J. J.; Wang, Y.; Peng, X. Colloidal Two-Dimensional Systems: CdSe Quantum Shells and Wells. Angew. Chem. Int. Ed. 2003, 42, 5035-5039.


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