5. Marco práctico
5.2 Narración comentada
A wide part of the PhD activity has been dedicated to the Scanning Electon microscopy (S.E.M) morphological and metrological analysis of the realized SERS substrates. SEM allowed us to acquire, through a sample scan, surface images of the sample itself. A first fundamental parameter to take into account in a SEM imaging is the acquired image contrast between exposed and not exposed zones. Infact, these zones have to be characterized by different depths respect to the surrounding surface. As an example Fig. 6.25 a shows a SEM image of a polymeric Thue Morse PQC of circles in which is clearly detectable a good contrast between exposed and not exposed zones; on the other hand, Fig 6.25 b shows a SEM image of a AgNWs based surface in which a good contrast is detectable between nanowires and glass substrate.
Resist spin-coating
Styrene Methyl Acrilate based Resist
GDSII PATTERNS of a Thue
Morse PQCs and a PCs Raith 150 EBL SYSTEM @ ICIB -CNR
Developing Positive resist Au deposition Before LIFT-OFF After LIFT-OFF Ebl nanopatterning Thermal evaporation E-beam evaporation SEM inspection &
125 Fig. 6.25: a) S.E.M image of a Thue Morse arrangement of cylindric holes denoting a very good contrast between exposed (light) and not exposed (dark) areas; b) SEM image of a AgNWs based surface in which a good contrast is detectable between nanowires and glass substrate.
Once acquired each image, a metrological analysis of the image is performed to detect all the relevant elements. Such two-dimensional analysis allows any measure in two dimensions: distances between points but even corners or circumference areas can be detected (Fig.6.26 a- b).
Fig.6.26 S.E.M images of a) a periodic PC of triangles with period 300nm in which distance between two points is taken and b) 8fold PQC of circles with diameter135nm in which the diameter of a single circle is provided. Both the structures shown are in polymer
Morphological and metrological characterization of AgNWs based SERS substrates
The following Fig.s (Fig. 6.27) show examples of SEM images and metrological analysis applied to the AgNWs surfaces obtained as described in Section 6.1, taking into account the characteristic sizes of Nanowires. In particular:
-Diameter: d
a) b)
126 -Lenght: L
Fig 6.27: Scanning electron microscopy (SEM) AgNWs surface a) at MAG 1KX ; b) at 15KX to investigate the diameter d, ranging from 150 to 230 nm; c)at 5 KX to investigate NWs length L, ranging from 5 to 15 µm
We achieved a not uniform NanoWires distribution on the surface of the substrate (Fig. 6.27 a) as confirmation of the lack of reproducibility of those kind of substrates. From the SEM analysis we found Nanowires with diameter ranging from 150 to 230 nm and lengths ranging from 5 to 15 µm
Morphological and metrological characterization of Engineered SERS nanostructures
We realized both Periodic square based and triangular based Photonic Crystals of circlular and trianglular-shaped rods (Fig. 6.30), and Aperiodic Photonic Quasi-Crystals in a Thue Morse arrangement of circles and squares, and in a 8-fold arrangement of circles with different sizes (d), period (A), interparticle separations (s) and filling factors (FF),
a)
127 summarized in the following Tables Tab 6.1 and Tab 6.2. Filling factor reflects the ratio of the covered and the non-covered metal areas and was calculated for each fabricated nanostructure in the following ways (Fig. 6.28-6.29) taking into account that in PQCs case the number of gold nanoparticles (N) was counted in the specific case of the total areas involved (ATOT) considered:
Periodic square based PCs of circles d=diameter; r=d/2 a=period
Total area= πr2
Filling Factor= (πr2)/a2
Periodic square based PCs of triangles d=side size
a=period
Total area= √ ⁄
Filling Factor= ( √ ⁄ )/a2
Periodic triangular based PCs of cylinders d=diameter; r=d/2 a=period
Total
area= √ ⁄ ⁄
Filling Factor= ⁄ √ ⁄ ⁄
Fig. 6.28: Filling factor evaluations for the three families of Periodic PCs realized
Fig. 6.29: Filling factor evaluations for the three families of Quasi-crystals realized 8-fold QPC of cylinders
d=diameter; r=d/2 a=period
ATOT=Total Area
involved
N=n° gold
nanoparticles
Gold Total covered area= Filling Factor= ATOT
Thue Morse QPC of squares d= side size G=period ATOT=Total Area
involved
N=n° gold
nanoparticles
Gold Total covered area= Filling Factor= ATOT
Thue Morse QPC of cylinders d=diameter; r=d/2 G=period
ATOT=Total Area involved
N=n° gold nanoparticles Gold Total covered area= Filling Factor= ATOT
128 Fig 6.30: Graphic representation of periodic PCs realized: a) Square based arrangement of circles; b)Square based
arrangement of triangles; c) Triangular based arrangement of circles
Tab. 6.1: Summary of the periodic Photonic Crystals realized
PERIODIC STRUCTURES SHAPE ARRANGEMENT SIZE
(d)
PERIOD (A) INTER-PARTICLE SEPARATION (s) FILLING FACTOR (FF)
Cylinder Square based 180nm 200nm 20nm 0,64
Cylinder Square based 180nm 240nm 60nm 0,44
Cylinder Square based 150nm 175nm 25nm 0,58
Cylinder Square based 150nm 200nm 50nm 0,44
Cylinder Square based 150nm 250nm 100nm 0,28
Cylinder Triangular based 100nm 150nm 50nm 0,4
Cylinder Triangular based 175nm 250nm 75nm 0,44
Triangle Square based 200nm 225nm 25nm 0,34
Triangle Square based 200nm 250nm 50nm 0,28
Triangle Square based 200nm 300nm 100nm 0,19
Triangle Square based 200nm 500nm 300nm 0,06
c)
b) a)
129 SEM and metrological analysis of the periodic nanostructures summarized in the previous table, realized firstly in polymer and then in metal deposited in the different ways previously described, are shown in Fig. 6.31-6.41. In each Fig. a comparison between different deposition methods is reported together with the starting structure made of air squared/circular/triangular-shaped holes nanopatterned in polymer.
Fig 6.31: SEM images of the PC pattern consisting of a Square based arrangement of circles with d=180nm, A=240nm, s=60nm on an area of 200 µm x 200 µm : (a) Air rods arranged into the ZEP matrix before thermal evaporation; (b,c) gold-based periodic SERS substrate after thermal evaporation Au (60nm) at different magnifications, 7.5 and 20 KX, respectively; (d) air rods arranged into the ZEP matrix before e-beam evaporation; (e,f) gold-based periodic SERS substrate after e-beam evaporation of Cr/Au (2nm/60nm) at different magnifications, 10 and 20 KX, respectively.
a)
b)
c)
e) d)
130 Fig 6.32: SEM images of the PC pattern consisting of a Square based arrangement of circles with d=180nm, A=200nm, s=20nm on an area of 200 µm x 200 µm (a) Air rods arranged into the ZEP matrix before thermal evaporation at magnification 13 KX; (b) gold-based periodic SERS substrate after thermal evaporation Au (60nm) at magnification 11K X.
Fig 6.33: : SEM images of the PC pattern consisting of a Square based arrangement of circleswith d=150nm, A=175nm, s= 25nm on an area of 200 µm x 200 µm: (a) Air rods arranged into the ZEP matrix before thermal evaporation at magnification 15 KX; (b) gold-based periodic SERS substrate after e-beam evaporation Cr/Au (2nm/50nm) at magnification 20K X.
a) b)
a)
f)
131 Fig 6.34: SEM images of the PC pattern consisting of a Square based arrangement of circles with d=150nm, A=200nm, s=50nm on an area of 200 µm x 200 µm (a) Air rods arranged into the ZEP matrix before thermal evaporation at magnification 10 KX; (b,c) gold-based periodic SERS substrate after e-beam evaporation of Au (60nm) at different magnifications, 10 and 20K X, respectively;
Fig 6.35: SEM images of the PC pattern consisting of a Square based arrangement of circles with d=150nm, A=250nm, s=100nm on an area of 200 µm x 200 µm (a) Air rods arranged into the ZEP matrix before thermal evaporation at magnification 10 KX; (b,c) gold-based periodic SERS substrate after thermal evaporation of Au (60nm) at different magnifications, 10 and 20KX, respectively; (d) Air rods arranged into the ZEP matrix before e- beam evaporation at magnification 10 KX (e,f) gold-based periodic SERS substrate after e-beam evaporation of Cr/Au (2nm/60nm) at different magnifications, 10 and 20 KX, respectively.
a)
d) b)
132 d)
e)
f) d)
133 Fig 6.36: SEM images of the PC pattern consisting of a Square based arrangement of triangles with d=200nm, A=225nm, s= 25nm on an area of 200 µm x 200 µm (a) Air triangles arranged into the ZEP matrix before thermal evaporation at magnification 10 KX; (b,c) gold-based periodic SERS substrate after thermal evaporation of Au (60nm) at different magnifications, 15 and 30KX, respectively; (d) Air triangles arranged into the ZEP matrix before e-beam evaporation at magnification 10 KX; (e)gold-based periodic SERS substrate after e-beam evaporation of Cr/Au (2nm/60nm) at magnification 20 KX. a) d) b) c) e)
134 Fig 6.37: SEM images of the PC pattern consisting of a Square based arrangement of triangles with d=200nm, A=250nm, s=50nm on an area of 200 µm x 200 µm (a) Air triangles arranged into the ZEP matrix before evaporation at magnification 20 KX; (b) gold-based periodic SERS substrate after thermal evaporation of Au (60nm) at magnification 30KX; (c) Air triangles arranged into the ZEP matrix before e-beam evaporation at magnification 10 KX; (d) gold-based periodic SERS substrate after e-beam evaporation of Cr/Au (2nm/60nm) at magnifications 20 KX.
Fig 6.38: SEM images of the PC pattern consisting of a Square based arrangement of triangles with d=200nm, A=300nm, s=50nm on an area of 200 µm x 200 µm (a) Air triangles arranged into the ZEP matrix before evaporation at magnification 20 KX; (b,c) gold-based periodic SERS substrate after thermal evaporation of Au (60nm) at different magnifications, 10 and 30KX, respectively; (d) Air triangles arranged into the ZEP matrix before evaporation at magnification 20 KX; (e,f) gold-based periodic SERS substrate after e-beam evaporation of Cr/Au (2nm/60nm) at different magnifications, 5 and 20KX, respectively.
a)
c) d)
b)
135 d)
e)
f) c)
136 Fig 6.39: SEM images of the PC pattern consisting of a Square based arrangement of triangles with d=200nm, A=500nm, s= 300nm on an area of 200 µm x 200 µm (a) Air triangles arranged into the ZEP matrix before thermal evaporation at magnification 20 KX; (b,c) gold-based periodic SERS substrate after thermal evaporation of Au (60nm) at different magnifications, 2.5 and 30KX, respectively.
Fig 6.40: SEM images of the PC pattern consisting of a Triangular based arrangement of circles with d=100nm, A=150nm, s= 50nm on an area of 200 µm x 200 µm (a) Air rods arranged into the ZEP matrix before thermal evaporation at magnification 15 KX; (b) gold-based periodic SERS substrate after thermal evaporation of Au (60nm) at magnifications 30KX.
a) b)
a)
c) b)
137 Fig 6.41: SEM images of the PC pattern consisting of a Triangular based arrangement of circles with d=175nm, A=250nm, s= 75nm on an area of 200 µm x 200 µm (a) Air rods arranged into the ZEP matrix before e-beam evaporation at magnification 10 KX; (b,c) gold-based periodic SERS substrate after e-beam evaporation of Cr/Au (2nm/60nm) at different magnifications, 10 and 20KX, respectively.
In Table Tab. 6.2 aperiodic Photonic Quasi-Crystals in a Thue Morse arrangement of circles and squares, and in a 8-fold arrangement of circles, with different sizes (diameter or side size d), lattice constant (G) and interparticle separations (minimum distance between the two closest rods-s) are reported. As in the previous case of periodic photonic structures, SEM and metrological analysis of the aperiodic nanostructures summarized in Table 6.2, realized firstly in polymer and then in metal deposited either thermally or e-beam, are shown in Fig. 6.41 - 6.53. In each Fig. a comparison between different deposition methods is reported together with the starting structure made of air rods holes nanopatterned in polymer.
a)
b)
138 APERIODIC STRUCTURES
SHAPE ARRANGEMENT SIZE (d) Lattice Constant (G) Minimum Interparticle Separation (s) FILLING FACTOR (FF) Square Thue-Morse 100nm 144nm 44nm 0,26 Square Thue-Morse 150nm 216nm 66nm 0.26 Square Thue-Morse 185nm 264nm 79nm 0,27 Square Thue-Morse 200nm 288nm 88nm 0,26 Square Thue-Morse 250nm 360nm 110nm 0.26 Square Thue-Morse 500nm 710nm 210nm 0.26 Cylinder Thue-Morse 100nm 144nm 44nm 0.19 Cylinder Thue-Morse 150nm 216nm 66nm 0.19 Cylinder Thue-Morse 250nm 360nm 110nm 0.20 Cylinder 8-Fold 100nm / 25nm 0.14 Cylinder 8-Fold 125nm / 30nm 0,18 Cylinder 8-Fold 150nm / 35nm 0.16 Cylinder 8-Fold 250nm / 70nm 0.18
Tab. 6.2: Summary of PQCs realized
Fig 6.42: SEM images of the PQC pattern consisting of a Thue Morse arrangement of squares with d=100nm, G=144nm on an area of 37 µm x 37 µm. (a) Air squares arranged into the ZEP matrix before thermal evaporation; (b,c,d) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured d=100nm, s=44nm at different magnifications, 5, 10 and 30 X, respectively.
139 Fig 6.43: SEM images of the PQC pattern consisting of a Thue Morse arrangement of squares with
d=150nm, A=216nm on an area of 55 µm x 55 µm. (a) Air squares arranged into the ZEP matrix before thermal
evaporation; (b,c,d) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured d=152nm, s=64nm at different magnifications, 2.5, 15 and 30 X, respectively.
a)
c) d)
c) d)
140 Fig 6.44: SEM images of the PQC pattern consisting of a Thue Morse arrangement of squares with
d=185nm, A=264nm on an area of 90 µm x 90 µm. (a) Air squares arranged into the ZEP matrix before thermal
evaporation; (b,c) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured d=178.8nm at different magnifications 15 and 30 X, respectively; (d) Air squares arranged into the ZEP matrix before e-beam evaporation; (e,f) gold-based aperiodic SERS substrate after e-beam evaporation of Cr/Au (2nm/60nm) with measured d=184.2 G=273.5 at magnifications 20 K X.
a) b) c) e) d)
141 Fig 6.45: SEM images of the PQC pattern consisting of a Thue Morse arrangement of squares with
d=200nm, A=288nm on an area of 74 µm x 74 µm. (a) Air squares arranged into the ZEP matrix before thermal
evaporation; (b,c,d) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured d=200nm, s=75nm at different magnifications, 5 and 23 KX, respectively.
a)
c) d)
b) f)
142 Fig 6.46: SEM images of the PQC pattern consisting of a Thue Morse arrangement of squares with
d=250nm, A=360nm on an area of 92 µm x 92 µm. (a) Air squares arranged into the ZEP matrix before thermal
evaporation; (b,c) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured G=364.7nm at different magnifications 3.5 and 30 X, respectively.
Fig 6.46: SEM images of the PQC pattern consisting of a Thue Morse arrangement of squares with
d=500nm, A=710nm on an area of 950 µm x 950 µm. (a) Air squares arranged into the ZEP matrix before
thermal evaporation; (b,c) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured G=708.4nm at different magnifications 2.5 and 10 X, respectively; (d) Air squares arranged into the ZEP matrix before e-beam evaporation; (e) gold-based aperiodic SERS substrate after e-beam evaporation of Cr/Au (2nm/60nm) with measured d=502.4nm at magnifications 10 KX
a)
b)
c)
143 Fig 6.48: SEM images of the PQC pattern consisting of a Thue Morse arrangement of circles with d=100nm,
A=144nm on an area of 150 µm x 150 µm. (a) Air rods arranged into the ZEP matrix before thermal evaporation;
(b,c,d) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured d=104nm, G=136nm, s=40nm at different magnifications, 10,15 and 30 KX, respectively.
a) b)
c)
144 Fig 6.49: SEM images of the PQC pattern consisting of a Thue Morse arrangement of circles with d=150nm,
A=216nm on an area of 160 µm x 160 µm. (a) Air rods arranged into the ZEP matrix before thermal evaporation;
(b,c,d) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured d=145nm, G=219nm at different magnifications 5, 15 and 30 KX, respectively.
a)
c) d)
b)
145 Fig 6.50: SEM images of the PQC pattern consisting of a Thue Morse arrangement of circles with d=250nm,
A=360nm on an area of 130 µm x 130 µm. (a) Air rods arranged into the ZEP matrix before thermal evaporation;
(b,c,d) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured d=250nm, G=364.7nm at different magnifications 2.5, 5 and 15 KX, respectively.
Fig 6.51: SEM images of the PQC pattern consisting of a 8-fold arrangement of circles with d=100nm on an area of 125 µm x 125 µm. (a) Air rods arranged into the ZEP matrix before thermal evaporation; (b,c,d) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured d=100nm at different magnifications 3.5, 10 and 30 KX, respectively. a) a) c) d) b) b)
146 Fig 6.52: SEM images of the PQC pattern consisting of a 8-fold arrangement of circles with d=125nm on an area of 155 µm x 155 µm. (a) Air rods arranged into the ZEP matrix before thermal evaporation; (b,c,d) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured d=125nm at different magnifications 3.5, 10 and 30 KX, respectively.
a)
c) d)
b)
147 Fig 6.53: SEM images of the PQC pattern consisting of a 8-fold arrangement of circles with d=150nm on an area of 185 µm x 185 µm. (a) Air rods arranged into the ZEP matrix before thermal evaporation; (b,c,d) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured d=140nm at different magnifications 3.5 and 30 KX, respectively
Fig 6.53: SEM images of the PQC pattern consisting of a 8-fold arrangement of circles with d=250nm on an area of 300 µm x 300 µm. (a) Air rods arranged into the ZEP matrix before thermal evaporation; (b,c,d) gold-based aperiodic SERS substrate after thermal evaporation Au (60nm) with measured d=245nm at different magnifications 3.5 and 30 KX, respectively.
a) b)
a)
b)
148 During the optimization of the fabrication protocol we have also found some problems in the procedures. As an example, Fig. 6.54 shows how the overdose effect on polymeric structures could propagates onto the metal counterpart seriously compromising the final result.
Fig. 6.54: SEM images of the PC pattern consisting of a Square based arrangement of circles with d=180nm, A=200nm, s=20nm on an area of 200 µm x 200 µm (a) Air rods arranged into the ZEP matrix before thermal evaporation at magnification 19 KX affected by a high overdose (Ad= 18 μC/cm2); (b) gold-based periodic SERS
substrate after thermal evaporation Au (60nm) at magnification 11K X clearly showing the overdose effect.
While, Fig. 6.55 shows common errors that can occur during the lift-off process due to two different potential motivations: it can happen, as an example, that not all the resist is removed, so it remains a Gold layer attached on the surface (Fig. 6.55 a), or the Gold Layer isn’t anchored to the glass/ITO surface, so during che lift-off process some nanoparticles could be missed (Fig. 6.55 b)
c)
149 Fig. 6.55: SEM images of the PC pattern consisting of (a) gold-based periodic SERS substrate after thermal evaporation Au (60nm) showing a not completely removed Gold layer during the lift-off process; (b) gold-based periodic SERS substrate after thermal evaporation Au (60nm) showing some nanoparticles missing due to an error in the lift-off process
References
[1] ZEON CORPORATION Electronics Materials Division, ZEP520A Technical Report,
ZEONREX® Electronic Chemicals
[2] Na Zhang, “Literature Research Report for Lift-Off Method”
150