U.S. patent application number 12/574379 was filed with the patent office on 2012-03-01 for optical devices for surface enhanced raman spectroscopy.
Invention is credited to David A. Fattal, Zhiyong Li, Qiangfei Xia.
Application Number | 20120050731 12/574379 |
Document ID | / |
Family ID | 45696850 |
Filed Date | 2012-03-01 |
United States Patent
Application |
20120050731 |
Kind Code |
A1 |
Fattal; David A. ; et
al. |
March 1, 2012 |
OPTICAL DEVICES FOR SURFACE ENHANCED RAMAN SPECTROSCOPY
Abstract
An optical device for surface enhanced Raman spectroscopy
includes a substrate, and at least one antenna established thereon.
The at least one antenna including at least two segments, where
each segment is formed of a metal having a predetermined volume and
a predetermined contact angle with respect to the substrate. A gap
is located between the two segments. The gap has a controllable
size such that the at least one antenna resonates at a
predetermined frequency that corresponds with the gap.
Inventors: |
Fattal; David A.; (Mountain
View, CA) ; Li; Zhiyong; (Redwood City, CA) ;
Xia; Qiangfei; (Palo Alto, CA) |
Family ID: |
45696850 |
Appl. No.: |
12/574379 |
Filed: |
October 6, 2009 |
Current U.S.
Class: |
356/301 ;
29/447 |
Current CPC
Class: |
G01J 3/44 20130101; Y10T
29/49865 20150115; G01N 21/658 20130101 |
Class at
Publication: |
356/301 ;
29/447 |
International
Class: |
G01J 3/44 20060101
G01J003/44; B23P 11/02 20060101 B23P011/02 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0001] This invention was made in the course of research partially
supported by a grant from The Defense Advanced Research Projects
Agency, Grant No. HR0011-09-3-0002. The U.S. government has certain
rights in the invention.
Claims
1. An optical device for surface enhanced Raman spectroscopy,
comprising: a substrate; and at least one antenna established on
the substrate, the at least one antenna including: at least two
segments, each segment formed of a metal having a predetermined
volume and a predetermined contact angle with respect to the
substrate; and a gap located between the two segments, the gap
having a controllable size such that the at least one antenna
resonates at a predetermined frequency that corresponds with the
gap, the gap size being controllable via heating until the at least
two segments and the substrate are in a state of equilibrium.
2. The optical device as defined in claim 1 wherein each of the at
least two segments has a predetermined geometry that is at least
partially controlled by the predetermined contact angle.
3. The optical device as defined in claim 2 wherein the substrate
includes a physical template or a chemical template that is
configured to at least partially control the predetermined geometry
of each of the at least two segments.
4. The optical device as defined in claim 1 wherein the gap has an
initial size and a final size, and wherein the final size is
smaller than the initial size.
5. The optical device as defined in claim 4 wherein the initial
size is formed via lithography, wherein the final size is formed
via heating of the two segments, and wherein the final size is
smaller than that achievable via lithography.
6. The optical device as defined in claim 1 wherein the gap has an
initial size and a final size, and wherein the final size is larger
than the initial size.
7. The optical device as defined in claim 1, further comprising at
least one other antenna established on the substrate, the at least
one other antenna including: at least two other segments, each
other segment formed of a metal having a predetermined volume and
an other predetermined contact angle with respect to the substrate,
wherein the other predetermined contact angle is different from the
predetermined contact angle of the at least one antenna; and an
other gap located between the two other segments, the other gap
having a controllable size such that the at least one other antenna
resonates at a predetermined frequency that corresponds with the
other gap.
8. (canceled)
9. A method for making an optical device for surface enhanced Raman
spectroscopy, comprising: selecting a metal having a predetermined
contact angle with respect to a substrate; establishing the metal
on the substrate to form at least two segments having a gap
therebetween, each of the at least two segments having an initial
geometry and a predetermined volume, and the gap having an initial
size; and heating the at least two segments, thereby i) altering
the initial geometry of each of the at least two segments to
achieve a final geometry while maintaining the predetermined volume
of each of the at least two segments, and ii) altering the initial
size of the gap to achieve a final size that is different than the
initial size.
10. The method as defined in claim 9 wherein establishing the metal
on the substrate to form the two segments is accomplished via a
lithography technique.
11. The method as defined in claim 9 wherein heating is
accomplished for a time sufficient for the at least two segments
and the substrate to reach an equilibrium state.
12. The method as defined in claim 11 wherein heating is
accomplished isothermally or transiently.
13. The method as defined in claim 9 wherein prior to establishing
the metal, the method further comprises forming a physical template
or a chemical template on a surface of the substrate, wherein the
physical template or chemical template is configured to at least
partially control the final geometry of each of the at least two
segments.
14. The method as defined in claim 9, further comprising tuning at
least a plasmonic resonance of the optical device by controlling
the predetermined contact angle.
15. The method as defined in claim 9 wherein heating causes the at
least two segments to reduce the initial gap size.
16. The method as defined in claim 9 wherein heating causes the at
least two segments to increase the initial gap size.
17. An optical system for surface enhanced Raman spectroscopy,
comprising: an optical device, including: a substrate; and at least
one antenna established on the substrate, the at least one antenna
including: at least two segments, each segment formed of a metal
having a predetermined volume and a predetermined contact angle
with respect to the substrate; and a gap located between the at
least two segments, the gap having a controllable size such that
the at least one antenna resonates at a predetermined frequency
that corresponds with the gap, the gap size being dynamically
controllable via heating until the at least two segments and the
substrate are in an equilibrium state; a light source operatively
configured to direct light toward the gap; and a detector
operatively positioned to detect an enhanced Raman signal from a
material of interest positioned at a region proximate to the
gap.
18. The optical system as defined in claim 17, further comprising
at least one other antenna established on the substrate, the at
least one other antenna including: at least two other segments,
each other segment formed of a metal having a predetermined volume
and an other predetermined contact angle with respect to the
substrate, wherein the other predetermined contact angle is
different from the predetermined contact angle of the at least one
antenna; and an other gap located between the two other segments,
the other gap having a controllable size such that the at least one
other antenna resonates at a predetermined frequency that
corresponds with the other gap.
19. (canceled)
Description
BACKGROUND
[0002] The present disclosure relates generally to optical devices
for surface enhanced Raman spectroscopy.
[0003] Raman spectroscopy is used to study the transitions between
molecular energy states when photons interact with molecules, which
results in the energy of the scattered photons being shifted. The
Raman scattering of a molecule can be seen as two processes. The
molecule, which is at a certain energy state, is first excited into
another (either virtual or real) energy state by the incident
photons, which is ordinarily in the optical frequency domain. The
excited molecule then radiates as a dipole source under the
influence of the environment in which it sits at a frequency that
may be relatively low (i.e., Stokes scattering), or that may be
relatively high (i.e., anti-Stokes scattering) compared to the
excitation photons. The Raman spectrum of different molecules or
matters has characteristic peaks that can be used to identify the
species. As such, Raman spectroscopy is a useful technique for a
variety of chemical or biological sensing applications. However,
the intrinsic Raman scattering process is very inefficient, and
rough metal surfaces, various types of nano-antennas, as well as
waveguiding structures have been used to enhance the Raman
scattering processes (i.e., the excitation and/or radiation process
described above). This field is generally known as surface enhanced
Raman spectroscopy (SERS).
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] Features and advantages of embodiments of the present
disclosure will become apparent by reference to the following
detailed description and drawings, in which like reference numerals
correspond to similar, though perhaps not identical, components.
For the sake of brevity, reference numerals or features having a
previously described function may or may not be described in
connection with other drawings in which they appear.
[0005] FIG. 1 is a flow diagram of an embodiment of a method for
making an embodiment of an optical device;
[0006] FIGS. 2A and 2B are schematic side views illustrating a
metal established on a substrate (FIG. 2A), and the antenna formed
after heating of the metal (FIG. 2B);
[0007] FIGS. 3A and 3B are top views of FIGS. 2A and 2B,
respectively;
[0008] FIG. 4 is a schematic side view of a metal established on a
substrate prior to heating, where the contact angle is
illustrated;
[0009] FIGS. 5A and 5B are schematic side views illustrating
another embodiment of a metal established on a substrate (FIG. 5A),
and the device formed after heating of the metal (FIG. 5B);
[0010] FIGS. 6A and 6B are schematic top views of metal established
on a substrate (FIG. 6A), and the array of antennas formed after
heating of the metal (FIG. 6B); and
[0011] FIG. 7 is a schematic diagram of a system including an
embodiment of the optical device.
DETAILED DESCRIPTION
[0012] The embodiments of the method disclosed herein
advantageously enable control over the gap size during fabrication
of, and in some instances after fabrication of, an antenna used,
for example, in Raman spectroscopy. By controlling the materials
and geometries used during fabrication, the resulting gap size may
be readily controlled at least until equilibrium is reached. In one
embodiment, the method disclosed herein allows gap size dimensions
to be obtained that are otherwise believed to be unattainable via
other processes, such as, for example, via lithography techniques.
The smaller gap size enhances the electric field in the gap (i.e.,
the hot spot), and thus the electric field is much stronger than
that of the incident electromagnetic (EM) wave in a certain
frequency range around the resonant frequency of the antenna. When
a material of interest (or an object made of the material of
interest) is placed at the hot spot, the Raman scattering of this
material is greatly enhanced.
[0013] Referring now to FIG. 1, an embodiment of a method for
making an optical antenna is illustrated. Very generally, the
method includes selecting a metal that has a predetermined contact
angle with respect to a selected substrate, as shown at reference
letter A. The metal is established on the substrate such that at
least two segments are formed having a gap therebetween, as shown
at reference letter B. The at least two segments are heated, as
shown at reference letter C, which alters the geometry of the
antenna and the size of the antenna gap. It is to be understood
that the various embodiments of the method will be discussed
further herein in reference to FIGS. 2 through 6.
[0014] Referring now to FIGS. 2A and 2B and 3A and 3B, side and top
views of one embodiment of the method for forming an embodiment of
the device 10 are respectively depicted. At the outset of the
method, as shown in FIGS. 2A and 3A, both a substrate 12 and a
metal 14 are selected. Non-limiting examples of suitable substrate
12 materials include single crystalline silicon, germanium,
polymeric materials (acrylics, polycarbonates, polydimethylsiloxane
(PDMS), polyimide, etc.), metals (aluminum, copper, stainless
steel, alloys, etc.), quartz, ceramic, sapphire, silicon nitride,
silicon dioxide, and glass.
[0015] The metal 14 selected will depend, at least in part, upon
the substrate 12 selected (or vice versa). Generally, the metal 14
is a Raman active material (e.g., noble metals such as gold and
silver). Suitable Raman active materials include those metals whose
plasma frequency falls within the visible domain, and which are not
too lossy (i.e., causing undesirable attenuation or dissipation of
electrical energy). The plasma frequency depends on the density of
free electrons in the metal, and corresponds to the frequency of
oscillation of an electron sea if the free electrons are displaced
from an equilibrium spatial distribution. In the embodiments
disclosed herein, the metal 14 selected also has a predetermined
contact angle .theta. (see, for example, FIG. 4) with respect to a
surface S of the substrate 12. The contact angle .theta. is the
angle at which a liquid/vapor interface (of the metal 14) meets a
solid surface (e.g., substrate 12). It is to be understood that the
contact angle .theta. at least partially controls the final
geometry of the metal 14 (i.e., segments 18', shown in FIGS. 2B and
3B), and thus the selected substrate 12 and metal 14 depend, at
least in part, upon the desirable gap size and geometry of the
resulting antenna 16 (shown in FIGS. 2B and 3B).
[0016] Once the substrate 12 and metal 14 are selected, the metal
14 is established on the substrate surface S. The metal 14 is
initially established with an initial volume and an initial
geometry. It is to be understood that the initial volume is
conserved throughout the process, while the initial geometry is
changed. The initial geometry includes at least two segments 18
having a gap G therebetween. It is to be understood that the gap G
of the initial geometry has an initial gap size G.sub.I. As a
result of further processing (described further hereinbelow), the
initial gap size G.sub.I and the initial geometry of the segments
18 are altered so that a desirable final gap size G.sub.F and
geometry of the segments 18' (shown in FIGS. 2B and 3B) are
obtained. As shown in the embodiments of FIGS. 2A and 2B and 3A and
3B, the final gap size G.sub.F is larger than the initial gap size
G.sub.I.
[0017] Two segments 18, 18' are illustrated in each of the Figures
discussed herein. While not shown, it is to be understood that the
segments 18 may be positioned in a triangular arrangement, a flower
petal arrangement, or another multi-segment configuration. All of
the suitable segment configurations have the gap G positioned at
the center of the segments 18, 18'.
[0018] In one embodiment, the initial geometric pattern of the
segments 18 and the initial gap size G.sub.I are formed by
nanoimprint lithography, electron-beam lithography,
photo-lithography, extreme ultraviolet (EUV) lithography, X-ray
lithography, any other suitable lithography technique, or any other
pattern defining technique. In another embodiment, the initial
geometric pattern of the segments 18 and the initial gap size
G.sub.I are formed by depositing the desirable metal 14 and etching
the metal 14 to pattern the segments 18. In still another
embodiment, the initial geometric pattern of the segments 18 and
the initial gap size G.sub.I are formed by depositing the desirable
metal 14 and then using a lift-off technique to pattern the
segments 18 from the deposited metal 14. In yet other embodiments,
the initial geometric pattern of the segments 18 and the initial
gap size G.sub.I are formed by direct deposition using focused ion
beam or plating. The examples provided herein are illustrative and
are by no means an exhaustive list.
[0019] After the segments 18 (having the initial geometry) are
formed on the substrate surface S, at least the segments 18 are
exposed to heating to form the antenna 16 and the device 10 (shown
in FIGS. 2B and 3B). Heating may be accomplished isothermally
(e.g., via conventional heating in an oven or furnace) or
transiently (e.g., via pulsed laser heating). The temperature at
which, and amount of time during which, heating takes place may
vary, depending upon the selected metal 14 and the amount of metal
14 present.
[0020] Heating causes the metal segments 14, 18 to soften or melt,
thereby rending them pliable. As such, the minimum heating
temperature and/or time is at least sufficient to initiate melting
of the metal 14. In a non-limiting example, the minimum heating
time is about 100 nanoseconds.
[0021] The surface tension causes the molten metal 14 to flow,
thereby causing segments 18 to change shape in a predetermined
manner, thereby ultimately forming segments 18' and antenna 16. As
shown in FIGS. 2B and 3B, the substrate 12 and the metal 14 may be
selected so that upon heating, the metal 14 spreads out (length-
and/or width-wise) such that the resulting segments 18' are thinner
or flatter than segments 18, and the size of the gap G becomes
smaller than the initial gap size G.sub.I.
[0022] Heating may be achieved until a desirable final gap size
G.sub.F is achieved and/or until the metal 14 and substrate 12
reach a state of equilibrium. In other words, the surface energy
between the metal 14 and the substrate 12 reaches a minimum, and
the metal 14 does not move anymore. As such, upon reaching
equilibrium, heating no longer affects the geometry of the segments
18' (i.e., there is no appreciable effect on the gap size).
[0023] If the desired final gap size G.sub.F is equivalent to the
gap size when the metal 14 and substrate 12 are in an equilibrium
state, heating may be accomplished until a change in the geometry
is no longer observed. In this embodiment, the gap size is
predetermined, and is based upon the initial geometry, and the
metal 14 and substrate 12 material selection. In other embodiments,
however, heating may be stopped prior to reaching equilibrium. This
may be accomplished when the desired final gap size G.sub.F is
equivalent to a gap size that is larger than the size of the gap G
corresponding with the equilibrium state. It is to be understood
that since the gap size may be modified until equilibrium is
reached, the gap G may be dynamically controlled (i.e., changed
when desirable, even after initial fabrication) until equilibrium
is reached. As such, the device 10 may have one or more final gap
sizes G.sub.F and segment 18' geometries (i.e., they may be altered
one or more times) prior to reaching the equilibrium state. For
example, during device 10 fabrication, the initial gap size G.sub.I
may be 20 nm, and then heating may be accomplished so that the
final gap size G.sub.F is equivalent to 15 nm. This device 10 may
be used in Raman spectroscopy, and will resonate at a frequency
that corresponds with the resulting geometry of the segments 18'
and the final gap size. Since the device 10 is not yet at
equilibrium, it may be desirable to further alter the device 10 to
resonate at another frequency. In this particular example, the
device 10 may be reheated to alter the segment 18' geometry and gap
size. As discussed herein, such alterations may take place until an
equilibrium state is reached.
[0024] When it is desirable to control the gap G size dynamically,
it is to be understood that the equilibrium may be altered by
introducing, for example, a coating on the substrate 12 or a dopant
into the metal 14. The introduction of such materials changes the
surface tension, and as a result, changes the flow properties of
the metal 14 upon exposure to heating. These additional parameters
may be altered multiple times in order to dynamically control the
gap G size.
[0025] In one non-limiting example, two Au cubes, each 100 nm in
size (i.e., 100 nm length, width and height), are established
(e.g., via lithography) on a SiO.sub.2 surface 50 nm apart.
Exposing these cube structures 14, 18 to heating causes them to
melt into two spherical shaped particles 14, 18', each with a
radius of 63 nm (considering a contact angle of 140.degree. between
molten Au and SiO.sub.2). The gap G between the two structures 18'
will be reduced from 50 nm to 24 nm.
[0026] In another non-limiting example, two Ag cubes, each 100 nm
in size (i.e., 100 nm length, width and height), are established
(e.g., via lithography) on a sapphire (Al.sub.2O.sub.3) surface 50
nm apart. Exposing these cube structures 14, 18 to heating causes
them to melt into two spherical shaped particles 14, 18', each with
a radius of 64 nm (considering a contact angle of 130.degree.
between molten Ag and Al.sub.2O.sub.3). The gap G between the two
structures 18' will be reduced from 50 nm to 22 nm.
[0027] In a third non-limiting example, two Al cubes, each 100 nm
in size (i.e., 100 nm length, width, and height), are established
(e.g., via lithography) on a sapphire (Al.sub.2O.sub.3) surface 50
nm apart. Exposing these cube structures 14, 18 to heating causes
them to melt into two spherical shaped particles 14, 18', each with
a radius of 73 nm (considering a contact angle of 82.degree.
between molten Al and Al.sub.2O.sub.3). The gap G between the two
structures 18' will be reduced from 50 nm to 4 nm.
[0028] Referring now to FIGS. 5A and 5B, another embodiment of the
method of forming the device 10' is schematically depicted using
side views. As shown in FIG. 5A, the substrate 12 and metal 14 are
selected, and the metal 14 is established to form segments 18.
Unlike FIGS. 2A and 2B and 3A and 3B, in this embodiment, the
substrate 12, metal 14, and initial geometry are selected so that
when exposed to heating, the gap G increases in size. The
interaction between the materials 12, 14 causes the metal 14 to
essentially build upon itself (again, the metal volume does not
change), thus increasing in height/thickness and increasing the gap
G between the segments 18. As a result, in this embodiment of the
antenna 16', the segments 18' are thicker than the initial segments
18, and the final gap size G.sub.F between the segments 18' is
larger than the initial gap size G.sub.I.
[0029] If the desired final gap size G.sub.F is equivalent to the
gap size when the metal 14 and substrate 12 are in an equilibrium
state, heating may be accomplished until a change in the geometry
is no longer observed. In other embodiments, heating may be stopped
prior to reaching equilibrium. This may be accomplished when the
desired final gap size G.sub.F is equivalent to a gap size that is
smaller than the size of the gap G corresponding with the
equilibrium state.
[0030] In a non-limiting example, two Ag rectangular cubes, each
100 nm in length, 100 nm in width and 20 nm in height, are
established (e.g., via lithography) on a sapphire (Al.sub.2O.sub.3)
surface 50 nm apart. Exposing these rectangular cube structures 14,
18 to heating causes them to melt into two spherical shaped
particles 14, 18', each with a radius of 38 nm (considering a
contact angle of 130.degree. between molten Ag and
Al.sub.2O.sub.3). The gap G between the two structures 18' will be
increased from 50 nm to 74 nm.
[0031] As illustrated in at least some of the examples given
herein, whether the gap G is increased or decreased after heating
depends, at least in part, on the initial volume and aspect ratio
of the metal segment 14, 18. If the metal segment 14, 18 is narrow
and tall (i.e., the height is greater than both the length and
width), upon heating, the metal 14 tends to reduce in height and
become wider in length and/or width. As a result, in this
embodiment, the gap G decreases. However, if the metal segment 14,
18 is wide and short (i.e., the height is smaller than both the
length and width), upon heating, the metal 14 tends to increase in
height and become narrower in length and/or width. As a result, in
this embodiment, the gap G increases. The behavior of the metal 14
may be determined by knowing the metal volume (which is conserved
after heating), the contact angle .theta. of the metal 14 to the
substrate 12, and that after heating, the metal segment 14, 18'
will have a spherical shape (which, in some instances, is
truncated).
[0032] In any of the embodiments of the device 10, 10' disclosed
herein, a physical or chemical template may be incorporated in or
on the substrate 12 in order to partially control the final
geometry of the segments 18'. The addition of such template(s)
allows the re-shaping of the geometry from the initial shape to the
final shape to be guided in a particular manner. Examples of
physical templates include trenches or rails etched into the
substrate 12. Such physical templates define boundaries for the
final geometry of the segments 18'. Examples of chemical templates
include the addition of surfactants or other materials with
desirable wetting properties to the surface S in a predefined
manner. Since the metal 14 may react with the treated portions of
the surface S differently than the untreated portions of the
surface S, the chemical templates also define boundaries for the
final geometry of the segments 18'. As a non-limiting example, the
physical and/or chemical templates may be used to guide the metal
from segments 18 having a circular geometry to segments 18' having
an elliptical geometry.
[0033] FIGS. 6A and 6B illustrate the top views of an array 100
both before the metal segments 14, 18 are exposed to heating (FIG.
6A), and after the formation of the antennas 16 (FIG. 6B). While
antenna 16' is not shown in FIGS. 6A and 6B, it is to be understood
that the array 100 could be configured to include antenna 16 and/or
antenna 16'. As illustrated in FIG. 6A, multiple metal segments 18
are established on the substrate 12 using one of the techniques
previously described. The metal 14 used for a single pair of
segments 18 (i.e., those segments that ultimately form a single
antenna 16) is generally the same. However, the metal 14 used for
one pair of segments 18 (one antenna 16) may be different from the
metal 14 used for one or more other pairs of segments 18 (another
antenna 16). After exposure to heat, the metal 14 flows to form the
spherical shaped segments 18' and the antennas 16. As such, the
array 100 includes multiple antennas 16 incorporated onto a single
substrate 12. In the embodiment shown, each of the antennas 16 has
the same gap size. By selecting the same metal 14 for each of the
segments 18, the array 100 includes the plurality of antennas 16
that resonates at the same frequency. It is to be understood that
one or more of the antennas 16 may be formed to have a different
gap size than another of the antennas 16 and thus different
antennas 16 may be configured to resonate at different
frequencies.
[0034] The devices 10, 10' disclosed herein are suitable for use in
standard Raman detection procedures. The system 1000 for such a
procedure is shown schematically in FIG. 7 and includes the device
10 having a material of interest 20 (e.g., analyte molecules or
particles) positioned in or near the gap G. The system 1000 further
includes a stimulation/excitation light source 22 and a detector
24. The material of interest 20 is subsequently subjected to laser
excitation of suitable stimulating/exciting wavelengths from the
light source 22. This light excites the material of interest 20,
and the resulting Raman signals are detected using known
detector(s) 24.
[0035] The embodiments of the antennas 16, 16' disclosed herein
have dynamically controllable (until equilibrium is reached) gaps
G, and as such, the plasmonic properties and the magnitude of the
field strength between the gap G of the devices 10, 10' may be
tuned for a desirable application. This provides an element of
control both throughout fabrication, and in some instances, after
fabrication and use. Heating may also smooth out the surface of the
resulting antenna 16, 16'.
[0036] While several embodiments have been described in detail, it
will be apparent to those skilled in the art that the disclosed
embodiments may be modified. Therefore, the foregoing description
is to be considered exemplary rather than limiting.
* * * * *