Beams of Electrosprayed Nanodroplets for Surface Engineering

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.