Beams of Electrosprayed Nanodroplets
for Surface Engineering
Manuel Gamero

Castaño
Department of Mechanical and Aerospace Engineering
University of California, Irvine
Energetic Impact by Electrosprayed Nanodroplets
2
•
Electrosprayed
nanodroplets
fill the unexplored size range between
ionic and macroscopic projectiles (between approx. 2

3 nm and 1
micron). Potential for new discoveries.
•
The impact of
nanodroplets
is similar to the hypervelocity impact of
macroprojectiles
. However local equilibrium is not reached near the
impact, resulting in new and interesting phenomena.
•
Nanodroplets
may extend the size

related advantages of cluster ion
beams (low
q/m
→ high molecular fluxes and sputtering rates,
restricted damage depth, etc).
•
Unlike cluster ion beams,
electrospraying
is a point source and
therefore its beam can be focused on a
submicrometric
spot (good
for SIMS and micromachining applications).
How Do We Know the Droplet Size?
3
•
Extensive body of work on
electrospraying
in the cone

jet mode:
accurate scaling laws for the droplet diameter and charge.
•
J.
Fernández
de la Mora, I.G. Loscertales,
The current transmitted through an electrified conical meniscus
.
Journal of Fluid Mechanics 260,155

84 (1994).
•
F. J. Higuera,
Flow rate and electric current emitted by a Taylor cone
. Journal of Fluid Mechanics. 484, 303

327 (2003).
•
M. Gamero

Castaño
,
Energy Dissipation in
Electrosprays
and the Geometric Scaling of the Transition
Region of Cone

Jets
. Journal of Fluid Mechanics, 662, 493

513 (2010).
•
M. Gamero

Castaño
,
The Structure of Electrospray Beams in Vacuum
. Journal of Fluid Mechanics, 604,
339

368 (2008).
•
Experimental determination of particle velocities and
q/m
via time

of

flight.
How Do We Know the Droplet Size?
4
•
Measuring an individual droplet’s charge and diameter via retarding
potential analysis and induction charge detection in tandem.
I
E
(
nA
)
<
q/m
> (C/kg)
<
D
> (nm)
<N
m
>
V
ACC
(kV)
<
v
d
> (km/s)
<
E
m
> (
eV
)
P
(
GPa
)
t
P
(
ps
)
373
650
34.8
51660
20.1
5.11
53.1
19.9
6.8
9.1
3.44
24.1
9.0
10
253
1116
24.3
17520
20.1
6.70
91.2
34.1
3.6
9.1
4.51
41.4
15.5
5.4
•
Typical droplet and impact parameters (EMI

Im
ionic liquid)
I
e
electrospray current; <
q/m
> average droplet charge to mass ratio; <
D
> av. droplet diameter; <N
m
> av. number of molecules in
droplet; <
v
d
> av. droplet velocity; <
E
m
> av. molecular energy;
P
projectile stagnation pressure;
t
P
characteristic compression time
M. Gamero

Castaño
,
Retarding potential
and induction charge detectors in
tandem for measuring the charge and
mass of
nanodroplets
, Rev. Sci.
Instrum
.
80, 053301 (2009).
Sputtering of Si, SiC and B
4
C
5
Experimental Setup
M. Gamero

Castaño and M. Mahadevan,
Sputtering Yields of Si, SiC and B4C under Nanodroplet
Bombardment at Normal Incidence.
Journal of Applied Physics, 106, 054305 (2009).
M. Gamero

Castaño and M. Mahadevan,
Sputtering of Silicon by a Beamlet of Electrosprayed
Nanodroplets. Applied Surface Science.
255, 8556

8561 (2009).
Sputtering of Si, SiC and B
4
C
6
Bombarded Si Wafer
Photograph, profile and AFM image of a Si target bombarded for 15 minutes with a beamlet
of EMI

Im nanodroplets at 14.1 kV acceleration voltage, and an electrospray current of 373
nA (34.8 nm average droplet diameter, 4.28 km/s impact velocity, 37.2 eV molecular kinetic
energy)
Sputtering of Si, SiC and B
4
C
7
Sputtering Yields and Sputtering Rates
•
The maximum sputtering yields for Si,
SiC
and B
4
C are 2.32, 1.48 and 2.29 atoms per molecule respectively. For
a comparison with atomic ion beams, the sputtering yields of Si,
SiC
and B
4
C bombarded by Argon at normal
incidence and 500
eV
are 0.4, 0.8, and 0.2 atoms per ion.
•
The maximum
nanodroplet
sputtering rates for Si,
SiC
and B
4
C are 0.448, 0.172, and 0.170
m
m/min. The
associated current densities are 9.26x10

3
, 1.55x10

2
and 1.33x10

2
mA
/cm
2
respectively. A broad

beam
Ar
source operates at a current density of 2
mA
/cm
2
and 500 V, and has sputtering rates of 0.060, 0.062, and 0.011
m
m/min for Si,
SiC
and B
4
C.
•
A
multiemitter
electrospray source with a density of1600 emitter/cm
2
would have a current density of the order
of 0.4
mA
/cm
2
;
&
its sputtering rate would be larger than that of a single emitter by a factor of the order of 40, and
the overall improvement with respect to IBM would be between 120 and 600.
&
Luis F. Velásquez

García, Akintunde I. Akinwande, Manuel Martínez
–
Sánchez, “A Planar Array of Micro

Fabricated
Electrospray Emitters for Thruster Applications”, Journal of Microelectromechanical Systems, 15, 1272

1280 (2006)
Pressure Induced Amorphization of Silicon
8
Surface Morphology and Sputtering Yield at
Increasing Impact Energy
9
Si targets bombarded at
increasing acceleration voltage
9.5 kV
11.0 kV
12.5 kV
15.5 kV
14.0 kV
17.5 kV
Sputtering yield as a function of
acceleration voltage
EBSD & HRTEM Confirmation of Amorphous Si Layer
10
a

Si
Si
b
IPF
colouring
Z0
Electron backscatter diffraction
mappings of bombarded surface
High resolution transmission
electron microscopy image
Pressure

Induced Amorphization Resulting from the
Absence of Local Equilibrium
11
•
No
amorphization
has ever been observed in macroscopic shock
compression of Si. The recovered material is always in the low
pressure, cubic diamond crystalline phase.
•
Si undergoes a high pressure crystalline phase transition at about
12
GPa
, evolving from the cubic diamond to the metallic
b

Sn
phase. The transition is reconstructive and has sluggish kinetics.
•
A likely explanation for the observed
amorphization
is that the
nanodroplet
impact compresses the cubic diamond phase beyond
its thermodynamic stability domain, but the brief compression time
impedes the growth of the
b

Sn
phase. The cubic diamond lattice
progressively deforms without being able to transform to
the
b

Sn
phase
,
and eventually loses its long

range order becoming
amorphous.
Amorphous Surface May Have Improved Wear Properties
12
Nanodroplet
impact on crystalline vs. amorphous surface
The absence of dislocations and grain boundaries in the
amorphous phase impede the propagation of cracks and the
fragmentation of the material
Future Work
13
•
Study the bombardment of colloid thrusters’ extracting electrodes by
energetic
nanodroplets
and molecular ions.
•
Study whether the observed
amorphization
of Si is reproduced on
other crystalline materials undergoing high pressure, reconstructive,
crystalline phase transitions (e.g.
Ge
,
SiC
,
GaAs
,
GaP
,
GaSb
,
InAs
,
InP
, etc
)
. New general, pressure

induced
amorphization
method.
•
Study of single
nanodroplet
impact, using RPA

ICD detector to
characterize projectile on its way to a target.
•
Multi

scale modeling, first

principles understanding of the impact.
•
3

D molecular profiling of organic surfaces via SIMS.
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