Debye Lecture 5
C. B. Murray
Nanowires
Size dependence of the band gap energy…
decreasing well
thickness (d)
decreasing dot d
InAs
Ellis and coauthors
J. Chem. Ed. 2001
InP CdSe
increasing E
Q dot
Alivisatos and coworkers
Science 1998
increasing E
Q wire
Eg
Q well
Quantum dot
3D confinement
1/dn
Quantum wire
2D confinement
Quantum well (layer)
1D confinement
How should the size dependences in wells,
wires, and dots compare quantitatively?
How should the size dependences of the band gaps (∆Eg values)
compare in wells, wires, and dots?
Particle-in-a-box models for kinetic
energy of confinement:
Quantum Well:
∆Eg = (h2/8d2)[1/me + 1/mh]
Cylindrical Quantum Wire:
∆Eg = 1.17(h2/8d2)[1/me + 1/mh]
Spherical Quantum Dot:
∆Eg = 2(h2/8d2)[1/me + 1/mh]
Quantum well
Quantum wire
Quantum dot
• Hypothesis: in plots of ∆Eg vs. 1/d2, the slope of a quantum-wire line
should be 0.585 times that of the corresponding quantum-dot line
A Laser Ablation Method for the Synthesis of
Crystalline Semiconductor Nanowires
Alfredo M. Morales and Charles M. Lieber
Science, vol 279, p 208 (1998)
Growth apparatus
Flow
controller
Pulsed
Laser
Lens
Sample
in Quartz
Tube
Tube Furnace
Cold
Finger
1-30 µm long
3-20 nm thick
Disordered, connected by round nod
Coated with SiO2 (10-20 nm)
Length limited only by growth
chamber
TEM of Si Nanowires
TEM
nanowires
Diffraction
contrast
High Res
TEM
0.3 nm
100 nm
10 nm
Convergent beam ED
The Growth Model
Laser
Ablation
Condensation
of Si on Ag
Supersaturation
and nanowire
growth
End of Growth
(wires exit
hot area)
TEM of Ge Nanowires
Ge and Fe
Ge only
9 nm
5 nm
1 nm
Silicon Nanowires: In-situ
Observation of Growth
viewing
direction
heated
substrate
Si2H6
110
111
112
110
111
112
Frances Ross, IBM Research
dark-field
image
Si nanowire
growth
showing wire and drop
geometry, facet
formation and tapering
to termination
Frances Ross, IBM Research
Solution-Liquid-Solid (SLS) Growth of Semiconductors…
Solution
t-Bu3M + EH3
protic
catalyst
3t-BuH
Liquid
In,Ga,Al
Bi, or Sn
Solid
InP
[111] growth direction
(ME)n
catalyst
particle
crystalline
semiconductor
(ME)
Trentler, Hickman, Goel, Viano, Gibbons,
Buhro Science 1995, 270, 1791
100 nm
• Unsuitable for quantum-wire studies…
• Most diameters are far too large (> 20 nm)
• Diameter distributions are far too broad
• Require monodisperse, small-diameter, catalyst nanoparticles
SLS growth of InP nanowires from monodisperse In nanoparticles…
Me3Si SiMe3
P
Me
Me
In
In
P
Me
Me
– SiMe4
203 oC, i-Pr2C6H4
In nanoparticles,
polymer stab.
Me3Si SiMe3
M.L. Steigerwald et al. 1991
• Polyhexadecene0.67-co- PVP0.33 was used as “surfactant” to stabilize both
In nanoparticles and InP nanowires
• Nanowire samples were grown using several sizes of monodisperse In
particles
• The nanowires formed dark-red dispersions that were stable indefinitely
• Nanowire samples were analyzed by TEM
H. Yu, J. Li, R.A. Loomis, L-w. Wang, W.E. Buhro Nature Mater. 2003, 2, 517
TEM images of InP nanowires: diameter control
9.9-nm In catalyst nanoparticles
4.5-nm InP nanowires
13.9-nm In
6.6-nm InP
21.2-nm In
11.0-nm InP
• Nanowire diameters scale with the initial catalyst-nanoparticle diameters
• Statistical analyses confirm fairly narrow nanowire diameter distributions
• The wires are near single crystals, and are 111 oriented
Absorption spectra of InP quantum wires and quantum dots…
quantum wires
quantum dots
Left: absorption spectra; Center: peaks
extracted by Gaussian fits and background
subtraction
Mićić, Nozik, and
coworkers Appl. Phys.
Lett. 1996
Plots of ∆Eg vs. 1/d2 for InP quantum dots and wires…
Recall prediction for the relative slopes of the lines: Awire/Adot = 0.585
InP quantum dots*
Adot = 6.1 ± 0.1 eV nm2
*Mićić, Nozik,
and coworkers
Awire/Adot = 0.62 ± 0.03
InP quantum wires
Awire = 3.8 ± 0.2 eV nm2
Thus, quantum
confinement is
weakened in the wires
to the expected extent
by the loss of one
confinement dimension
SLS growth of CdSe nanowires from Bi nanoparticles…
Cd(stearate)2
+
Se=P(n-Bu)3
240 – 300 oC, TOPO
Bi nanoparticles
(hexadecylamine)
adapted from
Xiaogang Peng
and coworkers
• No polymer stabilizer necessary to retain nanowire “solubility”
• Mean nanowire diameters varied with reaction temperature and catalyst
nanoparticle size
• Indefinitely stable dark-red dispersions
• Nanowire specimens were analyzed by TEM; typical lengths: several µm
• Statistical analyses: diameter std. dev. = ± 10 – 20% of mean diameter
Yu, Li, Loomis, Gibbons, Wang, Buhro JACS 2003, 125, 16168
TEM images of CdSe nanowires…
Absorption spectra of CdSe quantum wires…
d = 18.83 nm
d = 10.42 nm
d = 8.44 nm
d = 7.91 nm
d = 6.70 nm
d = 5.25 nm
selected
absorption
spectra
peaks extracted by
Gaussian fits and
background subtraction
Plots of ∆Eg vs. 1/d2 for CdSe quantum dots and wires…
Recall prediction for the relative slopes of the lines: Awire/Adot = 0.585
*Bawendi and coworkers
CdSe quantum dots*
Adot = 5.9 ± 0.4 eV nm2
Experiment for CdSe:
Awire/Adot = 0.53 ± 0.05
Experiment for InP:
Awire/Adot = 0.62 ± 0.03
CdSe quantum wires
Awire = 3.2 ± 0.2 eV nm2
T. J. Watson Research Center
Mechanisms of Particle and Wire formation
Particle Formation and Growth
Different Chemical Potential,
Injecting Condition, Ligand Concentration
Rocksalt
Structure
<100>
SA using Nanoparticles
as a Building Block
Discontinuous
Prorated Particles
Wire Formation and Growth
Thicker,
Continuous,
Crystalline
(100) PbSe Nanowires
acid − TOP, 170 − 250° C
Pb(CH 3COO) 2 + (C8H17) 3 PSe ⎯oleic
⎯⎯
⎯ ⎯ ⎯ ⎯⎯→ PbSe + ...
(100)
(100)
5 nm
PbSe Nanowires
(a)
40 nm
(b)
(c)
40 nm
40 nm
Solution Synthesized Nanorwires : Directed Self-Assembly.
PbSe:High yield, uniform structure,
band gap tunable between 0.75 and 0.5
Good diameter control..Improvement still
needed.
Deposited w/o field
Deposited DC field
Straight single crystal NWs
for transistors (Bulk PbSe
µ =1800 cm2V-1 S-1)
Directed self-assembly PbSe NWs
(110) PbSe nanowires
acid − HDA− TOP, 200° C
Pb(CH 3COO) 2 + (C8H17) 3 PSe ⎯oleic
⎯⎯
⎯⎯⎯⎯⎯
⎯→ PbSe + ...
HDA- hexadecylamine
Nanowires for thermoelectric applications
phonon
e
(110) PbSe nanowires
zig-zag (110) PbSe nanowires
are promising for for
thermoelectric applications
Shape evolution of PbSe Nanocrystals
Highly symmetric rock salt structure
100
111
Branched PbSe nanowires
(100)
PbSe nanowire-based heterostructures
acid − oleyl amine, HAuCl 4, 50° C
PbSe nanowire ⎯oleic
⎯⎯
⎯⎯⎯⎯⎯⎯⎯
⎯→ PbSe / Au