Support Information
The CdSe Qdots were synthesized according to our previous procedure1. For the preparation
of silica-coated Qdots, 20mg of CdSe Qdots passivated with triotylphosphine oxide (TOPO)/
tetradecylphosphonic acid (TDPA) was precipitated with ethanol and dried in vacuum. Then 50uL
of γ–aminopropyltriethoxy-silane (APTS) dissolved in 15mL of cyclohexane was added to the
precipitated Qdots, followed by addition of 4.46g of bis(2-ethylhexyl) sulfoccinate (AOT)
dissolved in 50mL of cyclohexane and stirring for 30min. Then 300-900uL of 30wt% NH4OH
was added and stirred for the desired time. Finally, 200uL of tetraethylorthosilicate (TEOS) was
added and stirred for different times to obtain uniform silica shells.
Structural and optical characterization of Qdots was carried out using several techniques. A
JEOL 2010F transmission electron microscope operated at 200kV was used for imaging and direct
analysis of the Qdots. A Perkin-Elmer PHI 5100 X-ray photoelectron spectrometer with Mg K
X-rays (1253.6eV) was used to collect XPS data from Qdots at binding energies ranging from 0 to
1250 eV, with a step size of 0.5eV, a dwell time of 30ms, and a pass energy of 89.45eV. X-ray
diffraction (XRD) patterns were obtained with a Philips APD 3720 system for information on
crystal structure and crystal size. The XRD pattern was collected in the step scan mode, with a 2θ
scan range of 15-80°, a step size of 0.01°, and a 1° grazing angle of incident X-rays. A JASCO
FP6500 system was used to collect PL spectra for the time evolution study and QY measurements.
Time resolved PL spectra were excited by the third harmonic (355nm) of a 5ns pulsed Quanta-ray
Nd:yttrium aluminum garnet (YAG) laser running at 10Hz, with a camera delay time of 10ns, a
gate width of 3ns and an increment interval time of 15ns.
The QY of silica-coated CdSe quantum dots dispersed in cyclohexane was determined by
comparing the integrated emission from Qdots to those from 9,10-diphenylanthracene (DPA),
fluorescein and rhodamine 6G for blue, green and red emitting samples, respectively. DPA,
fluorescein and rhodamine 6G were dissolved in cyclohexane, 0.1M NaOH and methanol,
respectively. The optical densities of the samples and standards solution, measured using a
Perkin-Elmer Lambda 800, were >0.01 and <0.06, respectively, at the excitation wavelength. The
absorbed intensity of the first excitonic peak was >0.01 and <0.02. Eq 1 was used to determine the
QYs of different Qdot samples 2.3.
1  10  ASt  2
I
QY  QYSt
 2 
A
1  10
 St I St
Eq.1
where QY and QYSt are quantum yields, A and ASt are absorbance values at the excitation
wavelength, η and ηSt are refractive indices of the solvents, and I and ISt are integrated emission
areas for the Qdot samples and the standard, respectively. The same excitation wavelength was
used for both the .sample and standard. The excitation wavelengths for samples are tabulated in
Table S1
1
L.Qian, D. Bera, P.H.Holloway, Nanotechnology, 19, 285702 (2008)
2
D.F., Eaton, Pure & Appl. Chem. 1988, 60, 1107.
3 D.
Bera , L. Qian, S. Sabui, S. Santra, P.H. Holloway, Optical Materials, 2008, 30, 1233.
Table S1: Quantum yields, excitation wavelengths and other related information for QY standards
Standard
QY
Solvent
DPA
90%
cyclohexane
Fluorescein
R6G
Excitation light[nm]
350
Color
Blue
95%
0.1M NaOH
450
Green
95%
methanol
500
Red
Table S2: Quantum yield and PL peak wavelength and shift of un-coated and silica-coated CdSe
quantum dots of four different sizes as shown in Fig 1b
PL position of uncoated sample (nm)
483
523
Quantum yield (%)
9
22
18
22
Δλ(nm) after silica coat
5
5
8
11
29
55
69
41
Quantum yield (%) of silica-coated
CdSe QDs
545
581
Table S3: Quantum yield and peak position of silica-coated CdSe quantum dots with different
times between adding NH4OH and TEOS. (a): NH4OH added followed immediately by TEOS; (b):
NH4OH added followed after 3mins by TEOS; (c): NH4OH added followed after 30mins by
TEOS.
Sample
CdSe
0minsa
3minsb
30minsc
quantum yields (%)
8.6
55
52
46
Δλ(nm) after
------
10
7
7
567
567
silica coated
Peak position (nm)
574
564
Table S4: Quantum yield and peak wavelength and shift from silica-coated CdSe quantum dots
with different amounts of TEOS and NH4OH.
Sample
CdSe
quantum yields (%)
7.5
Δλ(nm) after
----
QDs + 300uL NH4OH; QDs + 600uL NH4OH; QDs + 900uL NH4OH;
100uL TEOS
200uL TEOS
300uL TEOS
59
80
82
5
8
9
546
543
542
silica coated
Peak position (nm)
551