**3.2 Physical vapor deposition**

PVD is a vaporization deposition process for coating thin layers of materials. The material from a solid target is firstly vapored by a gaseous plasma or a hightemperature vacuum. Afterwards, the vapor is transferred to the surface of the substrate in vacuum or partial vacuum. Finally, it is condensed to produce thin films. The two most common kinds of PVD procedures are electron beam evaporation and sputtering. Electron beam evaporation is a process that generates an electron beam to evaporate atoms from the target into the gaseous phase, whereas sputtering creates vapor through bombardment of the target by ionized gas, typically argon (Ar). In both strategies, the produced vapor phase is subsequently condensed onto the surface of the substrate.

PVD has been shown to be an effective fabrication method in micro-/ nanomotors. Compared with template-assisted electrochemical deposition, PVD has some advantages, such as the ability to coat a wide range of materials, less fabrication processes, easier to operate, and more complicated geometries of micro-/ nanomotors can be fabricated. According to the deposition angles, there are two categories of PVD: conventional PVD and dynamical shadowing growth (DAG). In conventional PVD, the substrate is placed parallel to the target and the vapored metal flux is condensed almost vertically onto the substrate. DAG or glancing angle deposition (GLAD) is a PVD strategy in which the vapor is deposited onto a substrate at an oblique angle.

forward at velocities comparable to their nanowire counterparts. On the basis of sphere templates, PVD can be employed to synthesize not only spherical Janus motors but also versatile motors with various geometries. Valadares and co-workers studied a catalytic dimer comprising of a Pt half-sphere and a SiO2 sphere. The spheres were firstly deposited with a bilayer of Cr/Pt by using sputter machine, followed by an annealing process, during which the metallic half-shell formed a Pt

*(A) Schematic representation of the synthesis of bimetallic Janus micromotors by conventional PVD. (B) Synthesis of sphere dimers via thermal annealing. (C) Fabrication of asymmetric Pt/Au-coated catalytic micromotors by GLAD. (D) Fabrication of electrophoretic Pt-Au Janus nanoparticles by GLAD. (E) Synthesis procedures of L-shaped Si/Pt nanorod motors by GLAD. (F) Fabrication of catalytic micromotor comprising of a spherical SiO2 colloid with a TiO2 arm deposited asymmetrically with Pt. (G) SEM image of a Pt-Ag-Au shell micromotor synthesized by GLAD. Copyright 2010, ACS Publications. Copyright 2010, Wiley Online Library. Copyright 2010, AIP Publications. Copyright 2014, ACS Publications. Copyright 2007, ACS*

*Catalytic Micro/Nanomotors: Propulsion Mechanisms, Fabrication, Control, and Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.90456*

Relying on the substrate rotation during the deposition of an incident vapor and the self-shadowing effect, GLAD offers an easier way to synthesize Janus micro-/ nanomotors with complicated geometries. Zhao and his colleagues studied the asymmetric Pt/Au-deposited catalytic micromotors synthesized by GLAD. To get an asymmetric bimetallic deposition, a SiO2 microbeads-coated substrate was rotated to a polar angle after coating of an adhesive titanium (Ti) layer and an Au layer, the subsequent Pt coating left some of the Au layer exposed, as shown in **Figure 11C** [27]. The propulsion behavior could be regulated by changing the exposed area of the Au layer. Lee and co-workers synthesized a 30 nm Pt/Au Janus nanomotors by GLAD in which Au under fast substrate rotation was deposited onto

particle combined with the SiO2 sphere, as shown in **Figure 11B** [26].

*Publications. Copyright 2004, ACS Publications. Copyright 2013, RSC Publications.*

**Figure 11.**

**177**

Posner and co-workers reported the fabrication of a bimetallic spherical motor depending on the electrophoretic mechanism for motion, as shown in **Figure 11A** [25]. Firstly, the microspheres were half-deposited with Au by sputter machine. Afterwards, they were resuspended in water and coated again with Au in random orientation, which was repeated seven or eight times until the whole surface was fully deposited with Au. Finally, the Au-deposited spheres were half-deposited with Pt, which produced bimetallic spherical Janus micromotors that are able to propel

*Catalytic Micro/Nanomotors: Propulsion Mechanisms, Fabrication, Control, and Applications DOI: http://dx.doi.org/10.5772/intechopen.90456*

#### **Figure 11.**

**3.2 Physical vapor deposition**

**Figure 10.**

substrate at an oblique angle.

**176**

condensed onto the surface of the substrate.

PVD is a vaporization deposition process for coating thin layers of materials. The

material from a solid target is firstly vapored by a gaseous plasma or a hightemperature vacuum. Afterwards, the vapor is transferred to the surface of the substrate in vacuum or partial vacuum. Finally, it is condensed to produce thin films. The two most common kinds of PVD procedures are electron beam evaporation and sputtering. Electron beam evaporation is a process that generates an electron beam to evaporate atoms from the target into the gaseous phase, whereas sputtering creates vapor through bombardment of the target by ionized gas, typically argon (Ar). In both strategies, the produced vapor phase is subsequently

*Examples of micro-/nanomotors grown by template-assisted electrochemical deposition. (A) Electrodeposited Ag-Au/Pt and (B) Au/Pt-CNT nanomotors in H2O2. (C, D) Polycarbonate membrane-assisted growth and SEM images of conical PANI/Pt microtubes, respectively. (E) Growth procedures of flexible metallic nanowires with polyelectrolyte hinges after membrane template electrodeposition. (F, G) Anodized AAO membrane-assisted growth and SEM (scanning electron microscopy) images of segmented microtubes, respectively. (H) SEM image of a hinged nanowire. (I) SEM image of a Au/Agflex/Ni nanomotor with flexible central Ag segment. Copyright 2008, Wiley Online Library. Copyright 2008, ACS Publications. Copyright 2011, ACS Publications. Copyright 2007, Nature Publishing Group. Copyright 2013, RSC Publications. Copyright 2010, ACS Publications.*

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

PVD has been shown to be an effective fabrication method in micro-/ nanomotors. Compared with template-assisted electrochemical deposition, PVD has some advantages, such as the ability to coat a wide range of materials, less fabrication processes, easier to operate, and more complicated geometries of micro-/ nanomotors can be fabricated. According to the deposition angles, there are two categories of PVD: conventional PVD and dynamical shadowing growth (DAG). In conventional PVD, the substrate is placed parallel to the target and the vapored metal flux is condensed almost vertically onto the substrate. DAG or glancing angle deposition (GLAD) is a PVD strategy in which the vapor is deposited onto a

Posner and co-workers reported the fabrication of a bimetallic spherical motor depending on the electrophoretic mechanism for motion, as shown in **Figure 11A** [25]. Firstly, the microspheres were half-deposited with Au by sputter machine. Afterwards, they were resuspended in water and coated again with Au in random orientation, which was repeated seven or eight times until the whole surface was fully deposited with Au. Finally, the Au-deposited spheres were half-deposited with Pt, which produced bimetallic spherical Janus micromotors that are able to propel

*(A) Schematic representation of the synthesis of bimetallic Janus micromotors by conventional PVD. (B) Synthesis of sphere dimers via thermal annealing. (C) Fabrication of asymmetric Pt/Au-coated catalytic micromotors by GLAD. (D) Fabrication of electrophoretic Pt-Au Janus nanoparticles by GLAD. (E) Synthesis procedures of L-shaped Si/Pt nanorod motors by GLAD. (F) Fabrication of catalytic micromotor comprising of a spherical SiO2 colloid with a TiO2 arm deposited asymmetrically with Pt. (G) SEM image of a Pt-Ag-Au shell micromotor synthesized by GLAD. Copyright 2010, ACS Publications. Copyright 2010, Wiley Online Library. Copyright 2010, AIP Publications. Copyright 2014, ACS Publications. Copyright 2007, ACS Publications. Copyright 2004, ACS Publications. Copyright 2013, RSC Publications.*

forward at velocities comparable to their nanowire counterparts. On the basis of sphere templates, PVD can be employed to synthesize not only spherical Janus motors but also versatile motors with various geometries. Valadares and co-workers studied a catalytic dimer comprising of a Pt half-sphere and a SiO2 sphere. The spheres were firstly deposited with a bilayer of Cr/Pt by using sputter machine, followed by an annealing process, during which the metallic half-shell formed a Pt particle combined with the SiO2 sphere, as shown in **Figure 11B** [26].

Relying on the substrate rotation during the deposition of an incident vapor and the self-shadowing effect, GLAD offers an easier way to synthesize Janus micro-/ nanomotors with complicated geometries. Zhao and his colleagues studied the asymmetric Pt/Au-deposited catalytic micromotors synthesized by GLAD. To get an asymmetric bimetallic deposition, a SiO2 microbeads-coated substrate was rotated to a polar angle after coating of an adhesive titanium (Ti) layer and an Au layer, the subsequent Pt coating left some of the Au layer exposed, as shown in **Figure 11C** [27]. The propulsion behavior could be regulated by changing the exposed area of the Au layer. Lee and co-workers synthesized a 30 nm Pt/Au Janus nanomotors by GLAD in which Au under fast substrate rotation was deposited onto an array of Pt nanoparticles generated by block copolymer micelle lithography, as shown in **Figure 11D** [28]. Both of the bimetallic Janus motors depend on the selfelectrophoresis mechanism for propulsion.

He and his colleagues demonstrated the synthesis of rotary silicon/Pt (Si/Pt) nanorods, Si/Ag nanorods, and L-shaped Si/Pt where they firstly applied GLAD to synthesize the Si nanorod backbone and then asymmetrically deposited a Pt or Ag layer on one side of the nanorod backbone with a geometric shadowing effect. The L-shaped backbone was fabricated by a high speed of azimuthal rotation of the substrate in the middle of oblique angle deposition, as shown in **Figure 11E** [29]. By monitoring the substrate rotation and the deposition angle, complex rolling Si/Ag springs can be synthesized. Gibbs and Zhao reported the rotary propulsion of a micromotor comprising of a SiO2 microbead and a titanium dioxide (TiO2) arm with asymmetric Pt deposition. The arms of the micromotors were synthesized on the closely packed microbeads at oblique angles. As such, the Pt is subsequently coated only on one side of the arms at no angle, which offers the asymmetric placement of the catalyst critical for propulsion, as shown in **Figure 11F** [13]. With substrate rotation and oblique vapor direction, the coating layer can cover a much bigger area of the sphere templates than that by using conventional vapor deposition. A bubblepropelled Pt-Ag-Au shell micromotor with a smaller opening was synthesized by GALD and subsequent wet chemical etching, as shown in **Figure 11G** [30].

#### **3.3 Rolled-up nanotech**

By combining an engineered strain gradient with the coated thin membranes, the membranes are able to roll into the required shapes when detached from the substrate. The rolled-up nanotechnology pioneered by Schmidt and co-workers applies strain engineering to form micro-/nanotubes from deposited thin films. A prestressed nanomembrane is coated onto a photoresist sacrificial layer patterned by photolithography, which is able to be alternatively etched by acetone. GLAD deposition is applied to guarantee accurate positioning and tube integration on a single chip. A proper control of the deposition rate and the substrate temperature, as well as the stress evolution during coating, creates the strain gradient desired for the rolling process.

methacrylate) (PMMA) sacrificial layer beneath the sputtered Pt layer or by H2O2 assisted lift-off of the Pt layer coated directly on a glass wafer. A transmission electron microscopy (TEM) grid template was applied to synthesize microtubes with a relatively uniform size. Despite the low cost and simple methods described above provide great possibilities for large-scale yield of microtubes, a major issue of these methods is the lack of morphology and accurate control of the size of the rolled-up microtubes. On the premise of simplifying the processes, future efforts

*Rolled-up nanotech. (A) Rolling-up of nanomembranes patterned with photoresist: (a, b) schematic illustration of a rolled-up microtube comprising of Pt/Au/Fe/Ti multilayers on a sacrificial photoresist layer and an array of rolled-up microtubes, respectively; (c) SEM image of a rolled-up microtube. (B) Rolled-up microtubes with GO as an outside layer. (C) Reversible rolling and unrolling of thermoresponsive polymeric Pt microtubes. (D) Particle-aided rolling process of nanomembrane upon a thermal dewetting treatment. Copyright 2009, Wiley Online Library. Copyright 2010, Wiley Online Library. Copyright 2012, ACS Publications. Copyright 2014, Wiley Online Library. Copyright 2013, Wiley Online Library.*

*Catalytic Micro/Nanomotors: Propulsion Mechanisms, Fabrication, Control, and Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.90456*

Magdanz and his colleagues reported a flexible thermoresponsive polymeric microjet resulting from the reversible folding/unfolding of the polymer at decreased and elevated temperatures (**Figure 12C**) [34]. Cooling of the Pt/polymer layers results in folding of the films and synthesis of microtubes in diameter 30 μm with a Pt inner layer as a catalyst, while warming leads to the unfolding of the microtubes. Hence, the rolling and unrolling procedures of the microtubes could be conducted reversibly by changing the temperature of the solution to start and stop the propulsion of the microtubes. The diameters of the microtubes synthesized by the rolled-

Li and his partners took advantage of the surface tension of nanodroplets as well as the intrinsic strain relaxation in the nanomembranes to reduce the diameters of the rolled-up tubes to hundreds of nanometers. A layer of Pt was coated onto a prestrained bilayer of SiO2/TiO2 or Si/Cr on a sacrificial PMMA layer, as shown in **Figure 12D** [35]. In this treatment, rapid thermal process (RTP) was applied to stretch the Pt layer to the isolated islands and the nanodroplets brought considerable surface tension for rolling. On the other hand, the removal of PMMA resulted in the detachment of the nanomembranes. The synthesized microtubes exhibit

should be devoted to the better manipulation of the rolled-up technique.

higher velocities compared with those with a smoother Pt surface.

up nanotech are all in the range of microscale.

**Figure 12.**

**179**

The coated nanomembrane forms into a microtube once detached from the substrate by the dissolution of the sacrificial layer, as shown in **Figure 12A** [31, 32]. To avoid collapse of the rolled-up nanomembranes, the critical drying point is required to dry the synthesized microtubes. Microtubes with distinct opening diameters varying from 1 to 30 μm can be synthesized by modulating the built-in strain and the thickness of the nanomembranes. The lengths of the microtubes are in the range of scores of micrometers. Catalysts such as Pt consist of the inner wall of the microtubes by simply being coated onto the top layer of the nanomembranes. The wrinkle orientation of the detached membranes is defined by the different etching rates along the crystal axis and the crystal structure of the sacrificial layer.

Due to the high cost and the complex fabrication procedures of the rolled-up technique, considerable efforts have been devoted to simplifying the rolled-up procedures and decreasing its cost. Microtubes with outer layers of graphene oxide (GO) were synthesized by coating metal layers on GO nanosheets, as shown in **Figure 12B** [33]. Microscrolls with GO on the outer side and Pt at the inner surface were spontaneously synthesized upon sonication, resulting from material strain and weak bonding between GO layers. The diameter can be changed by modulating the thickness of the coated metal layers. A similar fabrication process of tubular microengines was demonstrated using accessible and low-cost fruit cells as support for the metallic layers. The tissue-based microengines can demonstrate extremely efficient bubble motion in the presence of H2O2. Zhao and his colleagues reported the synthesis of Pt microtubes by alternative dissolution of the poly(methyl

*Catalytic Micro/Nanomotors: Propulsion Mechanisms, Fabrication, Control, and Applications DOI: http://dx.doi.org/10.5772/intechopen.90456*

#### **Figure 12.**

an array of Pt nanoparticles generated by block copolymer micelle lithography, as shown in **Figure 11D** [28]. Both of the bimetallic Janus motors depend on the self-

He and his colleagues demonstrated the synthesis of rotary silicon/Pt (Si/Pt) nanorods, Si/Ag nanorods, and L-shaped Si/Pt where they firstly applied GLAD to synthesize the Si nanorod backbone and then asymmetrically deposited a Pt or Ag layer on one side of the nanorod backbone with a geometric shadowing effect. The L-shaped backbone was fabricated by a high speed of azimuthal rotation of the substrate in the middle of oblique angle deposition, as shown in **Figure 11E** [29]. By monitoring the substrate rotation and the deposition angle, complex rolling Si/Ag springs can be synthesized. Gibbs and Zhao reported the rotary propulsion of a micromotor comprising of a SiO2 microbead and a titanium dioxide (TiO2) arm with asymmetric Pt deposition. The arms of the micromotors were synthesized on the closely packed microbeads at oblique angles. As such, the Pt is subsequently coated only on one side of the arms at no angle, which offers the asymmetric placement of the catalyst critical for propulsion, as shown in **Figure 11F** [13]. With substrate rotation and oblique vapor direction, the coating layer can cover a much bigger area of the sphere templates than that by using conventional vapor deposition. A bubblepropelled Pt-Ag-Au shell micromotor with a smaller opening was synthesized by GALD and subsequent wet chemical etching, as shown in **Figure 11G** [30].

By combining an engineered strain gradient with the coated thin membranes, the membranes are able to roll into the required shapes when detached from the substrate. The rolled-up nanotechnology pioneered by Schmidt and co-workers applies strain engineering to form micro-/nanotubes from deposited thin films. A prestressed nanomembrane is coated onto a photoresist sacrificial layer patterned by photolithography, which is able to be alternatively etched by acetone. GLAD deposition is applied to guarantee accurate positioning and tube integration on a single chip. A proper control of the deposition rate and the substrate temperature, as well as the stress evolution during coating, creates the strain gradient desired for the rolling process. The coated nanomembrane forms into a microtube once detached from the substrate by the dissolution of the sacrificial layer, as shown in **Figure 12A** [31, 32]. To avoid collapse of the rolled-up nanomembranes, the critical drying point is required to dry the synthesized microtubes. Microtubes with distinct opening diameters varying from 1 to 30 μm can be synthesized by modulating the built-in strain and the thickness of the nanomembranes. The lengths of the microtubes are in the range of scores of micrometers. Catalysts such as Pt consist of the inner wall of the microtubes by simply being coated onto the top layer of the nanomembranes. The wrinkle orientation of the detached membranes is defined by the different etching rates along the crystal axis and the crystal structure of the sacrificial layer. Due to the high cost and the complex fabrication procedures of the rolled-up technique, considerable efforts have been devoted to simplifying the rolled-up procedures and decreasing its cost. Microtubes with outer layers of graphene oxide (GO) were synthesized by coating metal layers on GO nanosheets, as shown in **Figure 12B** [33]. Microscrolls with GO on the outer side and Pt at the inner surface were spontaneously synthesized upon sonication, resulting from material strain and weak bonding between GO layers. The diameter can be changed by modulating the thickness of the coated metal layers. A similar fabrication process of tubular microengines was demonstrated using accessible and low-cost fruit cells as support for the metallic layers. The tissue-based microengines can demonstrate extremely efficient bubble motion in the presence of H2O2. Zhao and his colleagues reported the synthesis of Pt microtubes by alternative dissolution of the poly(methyl

electrophoresis mechanism for propulsion.

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

**3.3 Rolled-up nanotech**

**178**

*Rolled-up nanotech. (A) Rolling-up of nanomembranes patterned with photoresist: (a, b) schematic illustration of a rolled-up microtube comprising of Pt/Au/Fe/Ti multilayers on a sacrificial photoresist layer and an array of rolled-up microtubes, respectively; (c) SEM image of a rolled-up microtube. (B) Rolled-up microtubes with GO as an outside layer. (C) Reversible rolling and unrolling of thermoresponsive polymeric Pt microtubes. (D) Particle-aided rolling process of nanomembrane upon a thermal dewetting treatment. Copyright 2009, Wiley Online Library. Copyright 2010, Wiley Online Library. Copyright 2012, ACS Publications. Copyright 2014, Wiley Online Library. Copyright 2013, Wiley Online Library.*

methacrylate) (PMMA) sacrificial layer beneath the sputtered Pt layer or by H2O2 assisted lift-off of the Pt layer coated directly on a glass wafer. A transmission electron microscopy (TEM) grid template was applied to synthesize microtubes with a relatively uniform size. Despite the low cost and simple methods described above provide great possibilities for large-scale yield of microtubes, a major issue of these methods is the lack of morphology and accurate control of the size of the rolled-up microtubes. On the premise of simplifying the processes, future efforts should be devoted to the better manipulation of the rolled-up technique.

Magdanz and his colleagues reported a flexible thermoresponsive polymeric microjet resulting from the reversible folding/unfolding of the polymer at decreased and elevated temperatures (**Figure 12C**) [34]. Cooling of the Pt/polymer layers results in folding of the films and synthesis of microtubes in diameter 30 μm with a Pt inner layer as a catalyst, while warming leads to the unfolding of the microtubes. Hence, the rolling and unrolling procedures of the microtubes could be conducted reversibly by changing the temperature of the solution to start and stop the propulsion of the microtubes. The diameters of the microtubes synthesized by the rolledup nanotech are all in the range of microscale.

Li and his partners took advantage of the surface tension of nanodroplets as well as the intrinsic strain relaxation in the nanomembranes to reduce the diameters of the rolled-up tubes to hundreds of nanometers. A layer of Pt was coated onto a prestrained bilayer of SiO2/TiO2 or Si/Cr on a sacrificial PMMA layer, as shown in **Figure 12D** [35]. In this treatment, rapid thermal process (RTP) was applied to stretch the Pt layer to the isolated islands and the nanodroplets brought considerable surface tension for rolling. On the other hand, the removal of PMMA resulted in the detachment of the nanomembranes. The synthesized microtubes exhibit higher velocities compared with those with a smoother Pt surface.
