In this study, we have developed an effective amino passivation process for quantum dots (QDs) at room temperature and have investigated a passivation mechanism using a photo-assisted chemical method. As a result of the reverse reaction of the H2O molecules, the etching kinetics of the photo-assisted chemical method increased upon increasing the 3-amino-1-propanol (APOL)/H2O ratio of the etching solution. Photon-excited electron-hole pairs lead to strong bonding between the organic and surface atoms of the QDs, and results in an increase of the quantum yield (QY%). This passivation method is also applicable to CdSe/ZnSe core/shell structures of QDs, due to the passivation of mid-gap defects states at the interface. The QY% of the as-synthesized CdSe QDs is dramatically enhanced by the amino passivation from 37% to 75% and the QY% of the CdSe/ZnSe core/shell QDs is also improved by ∼28%.

Colloidal semiconductor quantum dots (QDs) exhibit useful characteristics such as broad excitation and narrow and symmetrical fluorescence spectra, high-photostability and long emission intensity decays and are therefore promising materials for applications such as biological labels, optical switches, light-emitting diodes, and photovoltaics.1–7 However, several unresolved issues still limit their industrial application, such as their toxicity, low quantum yields, and propensity for surface oxidation. In general, an inorganic core exhibits a low QY% due to defect states present on the surface, which function as recombination sites for excited electrons.8–10 Consequently, the surface of the QDs is easily oxidized when exposed to oxygen and/or hydroxide containing environments. As a preventative measure, the internal inorganic core is surrounded by an external shell of inorganic or organic ligands. For example, core/shell structures can be constructed from CdS, ZnS or ZnSe as the inorganic material and TOPO/TOP (trioctylphosphine oxide/trioctylphosphine), alkylamine, pyridine as the outer organic ligands.5,6,11–16 As a result of greater tolerance to lattice mismatch and overcoating issues, shells composed of organic materials appear to form more adequate structures.

Among potential organic materials, the alkylamines have attracted considerable attention due to their ability for high-passivation of the CdSe QDs surface. Talapin et al. successfully replaced the surface ligand from TOPO with dodecylamine, which significantly increased the photoluminescence (PL) QY% of the QDs.11,17 Bullen et al., found that the chain length of the amine had no effect on the QY%. However, due to surface etching of the cadmium carboxylate groups, the PL emission peak exhibited a blue shift.18 Using etching methods, Li et al. have investigated size-tunable processes on CdSe QDs using a 3-amino-1-propanol (APOL)/water mixture solution at 80 °C.17 Despite the etching ability of the amine solution, the PL spectra, which corresponds to the size of the QDs, shows a stepwise blue shift after the etching process. For example, although the PL emission maximum peak remained at a constant wavelength during etching, the QY% exhibited both increase and decrease compared with the initial QY%. As the maximum peak exhibits a momentary blue shift, the QY% would also change. This behavior was observed repeatedly during the etching process. The variation in QY% appears to be due to the formation of an oxide layer as well as the passivation of surface defects by the amine group present on the surface of the QDs. However, it is important to note that neutral amines are not expected to exhibit a full exchange with the primary ligands. In order to understand the passivation effect of the amino group, an investigation into the exchange process of the primary ligand on the surface of the QDs is required. In a previous study, the mechanism behind the photo-induced chemical etching which also leads to passivation at the surface defects has been investigated.19 However, the passivation mechanism in relation to amino groups is yet to be studied to be studied.

Therefore, the aim of this study is to investigate the photo-assisted passivation of colloidal semiconductor QDs during with or without chemical etching of the QDs. By the application of light to the QDs, electron-hole pairs are generated on the surface of the semiconductor materials by the irradiation of photon energy. It is suggested that the generation of these electron-hole pairs produces energy which results in an interaction between the ligands and the surface of the QDs.

All chemical reagents used in this study were of analytical grade. In order to synthesize a narrow size distribution of the CdSe QDs, a microreactor system was employed as has been used in previous reports.20 Cd(CH3COO)2·H2O was dissolved in a solution of oleic acid and octadecene (ODE) at 180 °C under a flow of argon. A solution of Se powder dissolved in trioctylphosphine (TOP) was separately prepared. These two solutions were then mixed together and used as the source to produce CdSe. The solution was injected into a microreactor heated at 240 °C to produce TOP capped CdSe nanocrystals. The heating time was controlled by a specific flow rate and length for the heated microchannel. The CdSe/ZnS core/shell QDs were later synthesized using hot-injection methods.20–24 

To exchange the surface capping agent of the QDs for the amine passivation, APOL and H2O (v/v 10/1) solutions were loaded into the TOP capped QDs solution. The solution was then stirred intensively by a magnetic stirrer and simultaneously irradiated several times using a halogen lamp. The optical density (OD) of the APOL capped QDs solution was diluted by ethanol to create 0.09 OD solutions.

The absorption of the solutions was analyzed by UV-visible spectroscopy and photoluminescence emission spectra at room temperature in a 1 cm quartz cuvette using a fluorescence spectrometer with a Xe lamp. The PL QYs of the samples were determined by using rhodamine B and 6G as standard references (rhodamine B and 6G were dissolved in ethanol, QY: 75% and 95%, respectively) and were compared to the integrated PL intensities using standard procedures.

Figure 1(a) and (b) depict the UV-vis absorption and PL emission spectra of the the as-synthesized CdSe QDs dispersed in the APOL/H2O (v/v 10/1) solution as a function of the light irradiation time. A gradual blue shift of the first and second CdSe electronic absorption bands with increasing reaction time is clearly observed. The maximum intensity PL emission peak of the as-synthesized CdSe QDs were observed at a wavelength of c.a. 610 nm, this peak also exhibited a gradual blue-shift with increasing reaction time which is in good agreement with the absorption results. The related full width at half maximum (FWHM) of the as-synthesized QDs was estimated to be 40 nm, unfortunately, the FWHM was also observed to increase with increasing time to 52, 60, 63 and 76 nm after 30, 60, 90 and 120 min, respectively.

FIG. 1.

UV-vis absorption (a) and PL emission (b) spectra obtained for the CdSe QDs fabricated from the photo-assisted chemical method under 120 min irradiation in a APOL/H2O (v/v=10/1) solution.

FIG. 1.

UV-vis absorption (a) and PL emission (b) spectra obtained for the CdSe QDs fabricated from the photo-assisted chemical method under 120 min irradiation in a APOL/H2O (v/v=10/1) solution.

Close modal

Although the FWHM was observed to increase with time, the estimated QY% of the as-synthesized QDs, e.g., 37%, was observed to dramatically increase to 66%, 71%, 73%, and 75% for 30, 60, 90 and 120 min, respectively. This increase appears to be due to the surface defect passivation by the amine groups. The results are in good agreement with the previous study of Talapin et al.11 The as-synthesized QDs were capped by TOP, which can also be replaced by APOL, and the bonding energy of the CdSe surface and the amino group was increased by employing extra energy from the halogen lamp which is good agreement with the previous study.19 The blue shift observed in the PL emission is related to a decrease in the size of the QDs due to surface etching by the APOL/H2O solutions and is a demonstration of the quantum confinement effect. This result is in good agreement with the previous study by Li et al.17 However, the blue shift of the PL emission peak in this study is observed to be gradual. This differs from previous studies which have reported a stepwise blue shift. The stepwise blue shift is explained in terms of a “focusing and defocusing” in the growth mechanism. During the nanocrystalline growth, two distinct kinetic regimes occur: focusing is associated with the narrow size distribution and defocusing is associated with the broad size distribution. During this study, these two distinct kinetic regimes could not be observed. This is perhaps due to the high-velocity of the etching employed or that the etching occurs via a different mechanism.

In regard to the etching velocity, the dissolution of oxide layer appears to play a more critical role than the oxidation induced by the photon energy. Therefore, the v/v ratio of the APOL/H2O solution was changed from 10:1 to 5:1. Figure 2(a) depicts the UV-vis absorption spectra recorded for the CdSe QDs dispersed in the APOL/H2O (v/v 5/1) solution as a function of the light irradiation time. The first and second electronic absorptions of the as-synthesized CdSe was observed at wavelengths of 580 and 490 nm and these absorption peaks gradually exhibited a blue-shifted with increasing reaction time. This apparent blue shift of the first and second electronic absorptions was consistent with the gradual etching of the CdSe QDs. The etching time-dependent PL emission spectra with the corresponding absorption spectra are shown in Figure 2(b). The maximum PL emission intensity peak of the as-synthesized QDs observed a wavelength of around 590 nm which is slightly lower than that of the previous samples but much closer to those of the previous study by Li et al.17 

FIG. 2.

UV-vis absorption (a) and PL emission (b) spectra obtained for the CdSe QDs fabricated from the photo-assisted chemical method under 8 h irradiation in a APOL/H2O (v/v=5/1) solution (normalized PL emission peaks are found in the inset of Fig. (b)).

FIG. 2.

UV-vis absorption (a) and PL emission (b) spectra obtained for the CdSe QDs fabricated from the photo-assisted chemical method under 8 h irradiation in a APOL/H2O (v/v=5/1) solution (normalized PL emission peaks are found in the inset of Fig. (b)).

Close modal

The maximum intensity peak also exhibited a clear blue shift with increasing time however the FWHM showed only a slight increase (after 1 h).

The PL QY% of the as-synthesized QDs was estimated to be 19.8 % and the QY% showed a considerable decrease to 12.9 % after 1 h of photo-induced chemical etching. However, the QY increased with increasing reaction time, reaching 26.4 % by 8 h. This accompanied with the observed increase in the QY% are in good agreement with previous results.19 In order to provide a clearer picture of the photo-chemical etching process, the FWHM and the QY are plotted in Figure 3. By increasing the v/v ratio of the APOL/H2O solution from 10/1 to 5/1, the etching velocity exhibited approximately a four-fold decrease, which can be explained using a combination of the oxidation and dissolving processes. When the QDs are irradiated by the photon energy, electron-hole pairs are generated and oxidize at the surface of the QDs. The oxidation layer is subsequently etched-off by the APOL/H2O solution. In the presence of excess H2O however, a reverse reaction occurs which is outlined in the following reaction schematic:

CdO+2HOCH23NH2+HOCH23NH32SeO32[Cd(NH2CH23OH)4]2+SeO32+H2O
FIG. 3.

Summarized FWHM and the QY% of the CdSe QDs fabricated from the photo-assisted chemical method under 8 h irradiation in a APOL/H2O (v/v=5/1) solution.

FIG. 3.

Summarized FWHM and the QY% of the CdSe QDs fabricated from the photo-assisted chemical method under 8 h irradiation in a APOL/H2O (v/v=5/1) solution.

Close modal

Although the reaction velocity of the amino related chemical etching was affected by the H2O concentration,17 the photo-assisted chemical etching process is much faster than that of the thermal reactions.25,26

In general, when the QDs consist of a broad size distribution, they exhibit a higher value for the FWHM than compared to that of the narrow distribution, followed with a lower QY%. If the change in QY% follows the “focusing and defocusing” mechanism, the QY% should exhibit a stepwise change. However, in this study even though the FWHM increased with time, the QY% also showed an increase. This tendency shows no change when altering the etching kinetics. This signifies that the change of QY% observed in this study follows a different mechanism than that reported by Li et al.17 During the etching process, two kinds of passivation occur: 1) passivation by the oxidized layers, e.g., CdO and/or SeO2, 2) passivation by the amino groups. In the case of passivation by the oxidized layer, the QY% will increase when the oxide layer forms on the surface of the QDs, and decrease when the oxide layer with the amino groups is removed. However, in the case of the passivation by the amino groups, the surface of the QDs will be capped by the amino group in competition with the oxidation. At this initial stage, the QY% should decrease due to unstable surface states. However, when the surface was fully capped by the amino groups, the surface oxide layer will be etched-off. These reactions simultaneously occur. Therefore, the passivation by the amino groups does not exhibit a stepwise change and the passivation effect will increase during the etching process due to the more dominant surface defect states for smaller QDs. Furthermore, the passivation ability could also depend on the bonding between the surface atoms of the QDs and the amino group. In this study, electron-hole pairs were excited on the QDs surface by irradiation which leads to stronger bonding on the surface. From the above results a reaction mechanism is proposed in Figure 4.

FIG. 4.

Schematic diagram and reaction mechanism for the chemical passivation of the QDs by the amino groups with and without the photo-assisted system.

FIG. 4.

Schematic diagram and reaction mechanism for the chemical passivation of the QDs by the amino groups with and without the photo-assisted system.

Close modal

In order to confirm the passivation effects of the photon energy assisted system, we have used CdSe/ZnSe core/shell QDs. Figure 5 shows the PL emission spectra, which were recorded for the CdSe/ZnSe core/shell fabricated in the presence of O2 and H2O species after 30 min of irradiation. The maximum intensity PL emission peak of the as-synthesized QDs was observed at c.a. 530 nm and the peak exhibited a blue-shift to 516 nm after a reaction of 30 min. The blue-shift appears to be a result of the oxidation of the core QDs due to the strong oxidation effects of the photon energy. However, the measured QY% of the as-synthesized QDs showed a dramatic increase to approximately 28.3% compared to the as-synthesized QDs. The surface of the CdSe core is surrounded by the shell, i.e., ZnSe, which means the passivation effects of the inorganic materials is achieved spontaneously. When the surface defects exist in the shell and/or at the interface of the core/shell (which are located in the mid-gap of shell materials) a recombination of the excited electrons occurs at the core and results in a decrease of QY. Therefore, the enhancement of QY% is attributed to be a result of the passivation of defect states at the surface.

FIG. 5.

PL emission spectra obtained for the CdSe/ZnSe core/shell QDs fabricated from the photo-assisted chemical method under 30 min irradiation in a APOL/H2O (v/v=10/1) solution.

FIG. 5.

PL emission spectra obtained for the CdSe/ZnSe core/shell QDs fabricated from the photo-assisted chemical method under 30 min irradiation in a APOL/H2O (v/v=10/1) solution.

Close modal

In this study, an effective passivation method for the surface of QDs has been investigated using photon energy. The QDs were etched-off by photo-assisted chemical etching, which results in a blue-shift of the PL emission peak. The etching kinetics increased with an increase in the ratio of the APOL/H2O solution due to the occurrence of a reverse reaction of the H2O molecules. In comparison to previous studies, a stepwise change of the QY% was not observed due to surface passivation of the amino groups. The electron-hole pair excited by the photon energy leads to strong bonding between the organic and surface atoms of the QDs. This reaction also affects CdSe/ZnSe core/shell QDs. The QY% of the surface passivated QDs was enhanced from 37% to 75%. In summary, a strong chemical bonding method has been investigated using a photo-assisted system and is applicable to other organic systems.

This work was supported by the Ministry of Trade, Industry & Energy (MOTIE) through the Encouragement Program for The Industries of Economic Cooperation Region. (No. R0004019). The authors declare no competing financial interest.

1.
J.
Hu
,
L. S.
Li
,
W.
Yang
,
L.
Manna
,
L.-w.
Wang
, and
A. P.
Alivisatos
, “
Linearly polarized emission from colloidal semiconductor quantum rods
,”
Science
292
,
2060
2063
(
2001
).
2.
Y.
Li
,
A.
Rizzo
,
R.
Cingolani
, and
G.
Gigli
, “
Bright white-light-emitting device from ternary nanocrystal composites
,”
Adv. Mater.
18
,
2545
2548
(
2006
).
3.
J.
Lee
,
V. C.
Sundar
,
J. R.
Heine
,
M. G.
Bawendi
, and
K. F.
Jensen
, “
Full color emission from II-IV semiconductor quantum dot-polymer composites
,”
Adv. Mater.
12
,
1102
1105
(
2000
).
4.
M.
Bruchez
,
M.
Moronne
,
P.
Gin
,
S.
Weiss
, and
A. P.
Alivisatos
, “
Semiconductor nanocrystals as fluorescent biological labels
,”
Science
281
,
2013
2016
(
1998
).
5.
A. K.
Gooding
,
D. E.
Gómez
, and
P.
Mulvaney
, “
The effects of electron and hole injection on the photoluminescence of CdSe/CdS/ZnS nanocrystal monolayers
,”
ACS Nano
2
,
669
676
(
2008
).
6.
W. K.
Bae
,
K.
Char
,
H.
Hur
, and
S.
Lee
, “
Singe-step synthesis of quantum dots with chemical composition gradients
,”
Chem. Mater.
20
,
531
539
(
2008
).
7.
S. H.
Kang
,
K.
Kumar
,
K. C.
Son
,
H. H.
Huh
,
K. H.
Kim
,
C.
Huh
, and
E. T.
Kim
, “
Pyrolysis synthesis of CdSe/ZnS nanocrystal quantum dots and their application to light-emitting diodes
,”
Korean J. Mater. Res.
18
,
379
383
(
2008
).
8.
M.
Kuno
,
J. K.
Lee
,
B. O.
Dabbousi
,
F. V.
Mikulec
, and
M. G.
Bawendi
, “
The band edge luminescence of surface modified CdSe nanocrystallites: Probing the luminescing state
,”
J. Chem. Phys.
106
,
9869
(
1997
).
9.
S.
Cordero
,
P.
Carson
,
R.
Estabrook
,
G.
Strouse
, and
S.
Buratto
, “
Photo-activated luminescence of CdSe quantum dot monolayers
,”
J. Phys. Chem. B
104
,
12137
12142
(
2000
).
10.
D. W.
Jeong
,
J. Y.
Park
,
H. W.
Seo
,
K. M.
Lim
,
S. T.
Yeon
, and
B. S.
Kim
, “
Investigation on Fe-Hf-B-Nb-P-C soft magnetic powders prepared by high-pressure gas atomization
,”
J. Korean Powder Metall. Inst.
23
,
348
352
(
2016
).
11.
D. V.
Talapin
,
A. L.
Rogach
,
A.
Kornowski
,
M.
Haase
, and
H.
Weller
, “
Highly luminescent monodisperse CdSe and CdSe/ZnS nanocrystals synthesised in a hexadecylamine-trioctylphosphine oxide-trioctylphosphine mixture
,”
Nano Lett.
1
,
207
211
(
2001
).
12.
B. O.
Dabbousi
,
J.
Rodriguez-Viejo
,
F. V.
Mikulec
,
J. R.
Heine
,
H.
Mattoussi
,
R.
Ober
,
K. F.
Jensen
, and
M. G.
Bawendi
, “
(CdSe)ZnS core-shell quantum dots: Synthesis and characterisation of a size series of highly luminescent nanocrystallites
,”
J. Phys. Chem. B
101
,
9463
9475
(
1997
).
13.
A.
Kortan
,
R.
Hull
,
R. L.
Opila
,
M. G.
Bawendi
,
M. L.
Steigerwald
,
P.
Carroll
, and
L. E.
Brus
, “
Nucleation and growth of cadmium selenide on zinc sulfide quantum crystallite seeds, and vice versa, in inverse micelle media
,”
J. Am. Chem. Soc.
112
,
1327
1332
(
1990
).
14.
C. F.
Hoener
,
K. A.
Allan
,
A. J.
Bard
,
A.
Campion
,
M. A.
Fox
,
T. E.
Mallouk
,
S. E.
Webber
, and
J. M.
White
, “
Demonstration of a shell-core structure in layered cadmium selenide-zinc selenide small particles by x-ray photoelectron and Auger spectroscopies
,”
J. Phys. Chem.
96
,
3812
3817
(
1992
).
15.
M. A.
Hines
and
P.
Guyot-Sionnest
, “
Synthesis and characterization of strongly luminescing ZnS-capped CdSe nanocrystals
,”
J. Phys. Chem.
100
,
468
471
(
1996
).
16.
J. J.
Li
,
Y. A.
Wang
,
W.
Guo
,
J. C.
Keay
,
T. D.
Mishima
,
M. B.
Johnson
, and
X.
Peng
, “
Large-scale synthesis of nearly monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsoprtion and reaction
,”
J. Am. Chem. Soc.
125
,
12567
12575
(
2003
).
17.
R.
Li
,
J.
Lee
,
B.
Yang
,
D. N.
Horspool
,
M.
Aindow
, and
F.
Papadimitrakopoulos
, “
Amine-assisted facetted etching of CdSe nanocrystals
,”
J. Am. Chem. Soc.
127
,
2524
2532
(
2005
).
18.
C.
Bullen
and
P.
Mulvaney
, “
The effects of chemisorption on the luminescence of CdSe quantum dots
,”
Langmuir
22
,
3007
3013
(
2006
).
19.
S. Y.
Joo
,
D. W.
Jeong
,
C. G.
Lee
,
B. S.
Kim
,
H. S.
Park
, and
W. B.
Kim
, “
Room-temperature processing of CdSe quantum dots with tunable sizes
,”
J. Appl. Phys.
121
,
223102
(
2017
).
20.
B.
Swain
,
M. H.
Hong
,
L.
Kang
,
B. S.
Kim
,
N. H.
Kim
, and
C. G.
Lee
, “
Optimization of CdSe nanocrystals synthesis with a microfluidic reactor and developmental of combinatorial synthesis process for industrial production
,”
Chem. Eng. J.
308
,
311
321
(
2017
).
21.
A. M.
Nightingale
and
J. C.
de Mello
, “
Microscale synthesis of quantum dots
,”
J. Mater. Chem.
20
,
8454
8463
(
2010
).
22.
H.
Nakamura
,
A.
Tashiro
,
Y.
Yamaguchi
,
M.
Miyazaki
,
T.
Watari
,
H.
Shimizu
, and
H.
Maeda
, “
Application of a microfluidic reaction system for CdSe nanocrystal preparation: Their growth kinetics and photoluminescenece analysis
,”
Lab on a Chip
4
,
237
240
(
2004
).
23.
C. G.
Lee
,
M.
Uehara
,
Y.
Yamaguchi
,
H.
Nakamura
, and
H.
Maeda
, “
Micro-space synthesis of core-shell-type semiconductor nanocrystals for thermosensing
,”
Bull. Chem. Soc. Jpn.
80
,
794
796
(
2007
).
24.
A.
Puzder
,
A. J.
Williamson
,
N.
Zaitseva
,
G.
Galli
,
L.
Manna
, and
A. P.
Alivisatos
, “
The effect of organic ligand binding on the growth of CdSe nanoparticles probed by Ab Initio calculations
,”
Nano Lett.
4
,
2361
2365
(
2004
).
25.
T.
Matsumoto
,
M.
Maeda
,
J.
Furukawa
,
W. B.
Kim
, and
H.
Kobayashi
, “
Si nanoparticles fabricated from Si swarf by photochemical etching method
,”
J. Nanopart. Res.
16
,
2240
(
2014
).
26.
H.
Mizuno
,
H.
Koyama
, and
N.
Koshida
, “
Oxide-free blue photoluminescence from photochemically etched porous silicon
,”
Appl. Phys. Lett.
69
,
3779
(
1996
).