**1. Introduction**

The rapid development of communication technologies from First-Generation (1G) to Sixth-Generation (6G) has gained enormous attention due to its emerging services like human-to-human (H2H), machine-to-machine (M2M), and human-to-machine (H2M) communication. These emerging services are induced by drivers like mobile Internet, Internet of Things (IoT), and tactile Internet (TI). The IoT envisions to fill the gap between the cyber and physical world [1]. It is defined as to interrelate every existing computing object around us such as, mobile devices, sensors, and actuators, over the Internet. Moreover, IoT technology provides data sharing and communication in the M2M environment. Recently, the TI, with the aim to enable haptic communications, has shifted the IoT paradigm to real-time interaction between H2M and revolutionized the next-generation communication technologies [2,3]. The TI is envisioned to empower H2M communication where a human can interact with machines in a virtual and physical environment, while experiencing the haptic sensations (touch and forces) in addition to traditional audio-video data [4]. Figure 1 depicts the technological evolution of the communication trends from 1G to 6G wireless communication.

Several international standard organizations, such as the international telecommunication union, the Third-Generation Partnership Project (3GPP), and the Institute of Electrical and Electronics Engineering (IEEE), are working to enable the existing and develop new network architectures to carry haptic data over the communication in real-time. The TI standard working group IEEE P1819.1 has already initiated and defined reference architecture, technical functions, and the definition of the TI [5]. Moreover, it also described

**Citation:** Zubair Islam, M.; Shahzad; Ali, R.; Hadier, A.; Kim, H.S. IoTactileSim: A Virtual Testbed for Tactile Industrial Internet of Things Services. *Sensors* **2021**, *21*, 8363. https://doi.org/10.3390/s21248363

Academic Editors: Zihuai Lin and Wei Xiang

Received: 10 November 2021 Accepted: 13 December 2021 Published: 15 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

standard use cases of the TI and corresponding strict requirements, including teleoperation, automotive, immersive virtual/augmented reality, internet of drones, interpersonal communication, live haptic broadcast, and cooperative automated driving. However, these use cases demand near real-time connectivity (ultra-reliable and ultra-responsive) for M2M and H2M communication. This type of real-time connectivity is termed as ultra-reliable and low latency communication (URLLC). The URLLC is one of the key services of the Fifth-Generation (5G) networks, along with enhanced mobile broadband and massive machine-type communication. Moreover, 3GPP has introduced the 5G new radio to increase reliability and minimize end-to-end (E2E) communication latency for the URLLC services. In Release 15, 3GPP describes the URLLC requirement with the reliability of 99.9% for a single 32-byte packet under 1ms latency [6]. Conclusively, 5G URLLC services are one of the potential enablers for the extreme requirements of the TI.

**Figure 1.** A taxonomy of the different emerging communication trends.

Moreover, these requirements become more critical for loss-intolerant and delaysensitive TI industrial and medical applications. For example, remote industrial management and the automation of industrial robots (sensitivity of control circuits) demand latency between 0.25–10 ms with a packet loss of ≤10−9 [7]. Therefore, supporting next-generation industrial applications, including immersive reality, holographic, and haptic/tactile communication, demands a 5G network with new physical and upper layer techniques to guarantee quality of service (QoS) and quality of experience (QoE) provisioning. Furthermore, the 6G technology paradigm promises to break the 5G network limitations and enable them to virtualize human skills and transfer them from one place to another within 1ms through 6G native artificial intelligence (AI) network architecture. In-depth work on 5G URLLC services, beyond the 5G and 6G communication network, is presented in these articles [8–12]. Table 1 compares the connectivity requirements of the traditional and emerging tactile IIoT applications (adapted from [7]). The relationships between emerging technologies such as IoT, IIoT, Industrial Internet, Internet of Everything (IoE), TI, tactile IoT, tactile IIoT, Industry 4.0 and 5.0 are presented in Figure 2.

An in-depth discussion on conventional and emerging industrial is presented in [7], where the authors investigated the role of TI in the industrial environment, along with technical connectivity requirements of the TI industrial services. One of the vital use cases of the TI in the industrial domain is the bilateral/multilateral haptic-driven teleoperation systems. A teleoperation system consists of a human operator (master), teleoperator (slave), and a communication network that link the master to a slave domain, and enable the operator to interact with the teleoperator in the distant and inaccessible remote environment to perform complex tasks. The TI-based network provides bilateral communication to manage touch and actuation in real-time between the master and slave domain with a

focus to ensure QoS and QoE requirements. Haptic-enabled teleoperation systems have numerous applications in Industry 4.0, such as robotic automation, smart manufacturing, smart logistic, the mining industry, food industry, healthcare industry, and industrial management (controlling and monitoring). Contrary to the traditional application, Hapticenabled industrial applications demand high QoS and QoE, and depend on the nature of the application.

**Table 1.** Summary of the connectivity requirements for traditional IIoT and emerging tactile IIoT services.

**Figure 2.** An overview of the relation between IoT, IIoT, tactial IoT, tactile IIoT, Industry 4.0, and Industry 5.0.

One of the effective ways to investigate the tactile IIoT application requirements, performance, and testing the new solutions to ensure QoS and QoE, is to set up a virtual testbed similar to the real network. The testbed must allow us to utilize and maintain hardware and software virtually on a standard computer without purchasing them. In the literature, several recent articles have proposed testbeds to overcome the above-mentioned challenges. The work in [13] proposed a haptic system testbed to characterize and validate E2E haptic communication of different use cases of TI. The authors introduce a framework comprised of multiple sub-blocks that can be re-configured based on the nature of use cases, with a focus on minimizing cost and evaluation time. It also provides an option to integrate the testbed with the simulation platform through a connector interface to perform testing. Commonly, it is intended to offer an extensive range of haptic hardware, including sensors, actuators, and tactile interface boards. A testbed for tactile and kinesthetic data coding was proposed in [14] aligned with IEEE P1918.1 TI standard working group to improve and standardize haptic codec. The proposed haptic coding testbed is considered as a reference testbed with the aim to develop optimal data compression schemes to exchange tactile and kinesthetic information and enable human-in-the-loop TI services. The authors

also provide some reference tactile data traces, software, and hardware to evaluate newly developed kinesthetic and tactile codecs.

In [15], a framework for tactile cyber physical systems was proposed, which is specifically for physical remote environments and based on network simulator NS3. It provides an interface for robotic experiments, along with haptic communication modules. However, the authors ignored the extensibility of the proposed testbed for other haptic-driven applications. Similarly, the authors in [16,17] proposed a generic testbed framework for different TI use cases. A data-driven experiment setup was proposed in [16] to provide a common playground for testing haptic applications. The proposed haptic communication testbed at the Otto-von-Guericke University of Magdeburg (OVGU-HC) focused on providing experiment testbed for long-distance haptic-enabled teleoperation systems, in addition to small scale wireless haptic-driven applications. The OVGU-HC presents experiment automation and data collection utilizing experiment description language (DES-Cript). The proposed OVGU-HC did not work standalone, is a part of the MIoT-Lab, and is just used to gather hepatic experiment information. Moreover, it utilized domain-specific language DES-Cript [18], and did not provide an open-source facility to the research and development community.

The study in [17] presents a two-level classification of the TI applications based on controlled environment and master-slave integrations to develop a generic testbed, with a focus to ensure compatibility for all these classified applications, which is named as TI- eXtensible Testbed (XT). To demonstrate the potential of the TIXT, they discuss H2M haptic communication in the virtual and physical environment. However, they ignored the explanation on how to characterize the network impairments (delay, jitter, and packet losses) and investigate the performance of the haptic-driven IIoT application. Therefore, there is a strong need for a testbed that offers flexibility, scalability, open-source availability, tailored to examine network impairments, communication flow, and extensible for TI IIoT use cases. In this regard, we proposed a virtual testbed called IoTactileSim to investigate tactile IIoT services from QoS and QoE perspectives. The IoTactileSim employs Software Define Network (SDN) and edge computing at the core network to tactile industrial application. The following section presents the main contribution of the proposed IoTactileSim testbed.


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## *Edited by Fahmina Zafar and Anujit Ghosal*

To ensure a healthy lifestyle, fire safety and protocols are essential. The population boom, economic crunches, and excessive exploitation of nature have enhanced the possibilities of destruction due to an event of a fire. Computational simulations enacting case studies and incorporation of fire safety protocols in daily routines can help in avoiding such mishaps.

Published in London, UK © 2020 IntechOpen © stockphoto / iStock

Fire Safety and Management Awareness

Fire Safety

and Management Awareness

*Edited by Fahmina Zafar and Anujit Ghosal*


Preface

 It is an established nutritional fact that animal proteins are superior to vegetable proteins for the supply of essential amino acids. Milk is considered as a complete diet for an infant and contains essential nutrients for the development of young mammals. The substances in milk provide energy and antibodies that help protect against infection. Dairy animals of the present era are the outcome of thousands of years of breeding of undomesticated animals living in different parts of the world at different altitudes and latitudes, exposed to severe and extreme weather conditions. The present techniques used to produce milk from cows, goats, sheep, camels, and buffaloes started around 6000 years ago. The same species of animals are being maintained as dairy animals today. Most farmers are paid on the quality and composition of their milk. Whole milk, once approved for use, is pumped into storage silos where it undergoes pasteurization, homogenization, separation, and further processing. Milk is a highly perishable commodity because it is an excellent medium for the growth of microorganisms—particularly bacterial pathogens—that can cause spoilage as well as diseases in consumers. Milk processing allows the preservation of milk for days, weeks, or months and helps to reduce food-borne illness. An efficient milk marketing chain is one that enables farmers to receive at least 50% of the retail price of milk. The price of a product in the market is an important factor influencing consumer demand. When marketing a dairy product, the most important aspect of the strategy is determining the competition and audience. Through establishing these parameters, milk producers can help to flush out what aspect of a dairy business to highlight and where will be the most effective place to advertise to capture the target audience.

A large number of publications on milk production, processing, and marketing are available in the world market. The book under discussion has not been compiled to add yet another to such publications, rather it has been written using a novice format to meet the requirements of students, researchers, and policy makers working in different parts of the world in different environments in the fields of milk production, processing, and marketing.

> **Dr. Khalid Javed**  Professor, Department of Livestock Production, University of Veterinary and Animal Sciences, Lahore-Pakistan

**1**

**Chapter 1**

**Abstract**

Current Standing and Future

Challenges of Dairying in

Pakistan: A Status Update

*Muhammad Bilal and Hafiz Muhammad Nouman*

Pakistan is considered among the leading raw milk producing countries. Unlike the production systems in the developed countries, milk production systems in Pakistan represent smallholding with subsistence- or market-oriented-level farming followed by peri-urban or commercial-level farming. Historically, dairy sector has been owned and managed by the private sector. During the past two decades, new initiatives have been taken because of the active involvement of corporate private sector. These efforts have resulted in improvements like enlargement of herds and import of high-quality milk germ plasm, the productivity per animal, milk collection, processing and marketing, the supply of dairy inputs (machinery, equipment, feeds, semen, and elite dairy animals), and farmers knowledge, and skills on modern management practices. Conclusively, the dairy sector is performing at some sustainable level to meet the food requirements of the growing population and helping save a handful of foreign exchange. Yet, challenges like local replacements of high genetic potential dairy animals, health hazards of β-casein proteins, antibiotics and aflatoxins, and uneconomical operational costs facing the dairy industry in the near future need to be addressed. The main objective of this chapter is to identify the current trends in dairy industry of Pakistan and describe those factors, which can influence the sustainability and profitability of dairying in the near future.

**Keywords:** collection and processing, dairy inputs, food legislation, large peri-urban

Pakistan is considered among the leading raw milk producing countries. Unlike the production systems in the developed countries like United States of America and most of Europe, milk production systems in Pakistan have similar characteristics to the most developing countries of subcontinent. Characteristically, smallholding with subsistence- or market-oriented-level farming keeps the major share

Current dairying in Pakistan is a combination of both traditional and commercial methods of raising dairy animals, and producing milk and milk products. During last two decades, commercial farming with imported dairy inputs has increased and reached roughly about 1% of the total raw milk production. With the

(about 90%) followed by peri-urban or commercial-level farming [1].

dairy farming, profitability, sustainability

**1. Introduction**

*Muhammad Naeem Tahir, Roshan Riaz,*


**1. Introduction**

The plastids of algae and land plants are semi-autonomous organelles with their own genomes, complete with transcription and translation machinery [1] that arose from endosymbiosis between a eukaryotic ancestor and a photosynthetic cyanobacterium [2]. The plastid genome is a circular doublestranded DNA molecule that encodes 100–250 genes and is maternally inherited in angiosperms [3]. The size of chloroplast genomes varies between species, ranging from 107 kbp (Cathay silver fir, *Cathaya argyrophylla*) to 218 kbp (Geranium, *Pelargonium* spp.). The plastid genome is present in many copies inside the organelle and is protected from gene silencing pathways that typically lower transgene expression, thereby allowing high levels of foreign protein to accumulate, achieving 5–40% total soluble protein (TSP) [4] and up to 70% of total soluble protein in tobacco (*Nicotiana tabacum*) [5–7]. An even higher protein yield (>75% TSP) was recently achieved in tobacco transplastomic plants expressing a hyper-thermostable form of β-glucosidase cloned from the hyperthermophilic bacterium *Pyrococcus furiosus* [8]. Compared to nuclear transformation, plastid transformation has many advantages. First, the maternal inheritance of the chloroplast genome prevents genes from escaping through pollen grains in most plants and thus reduces the spread of transgenes in the environment and avoids issues with Genetically Modified Organisms (GMOs) [9]. Second, the multiple copies of the plastid genome contained within the organelle results in the high expression of foreign genes and the accumulation of the encoded proteins [10]. Third, since plastid transformation involves homologous recombination (HR) of the transgene into a so-called neutral site, the method largely avoids gene silencing caused by position effects [11]. Fourth, the chloroplast employs a prokaryotic gene expression system and allows the easy and simultaneous expression of polycistronic genes [12]. Last, the stacking of multiple transgenes can be performed in a single and highly efficient transformation event [13]. Plastid transformation is, therefore, gaining traction [14].

The main crop species used for plastid transformation are in the Solanaceae, including tobacco, potato (*Solanum tuberosum*), tomato (*S. lycopersicum*), eggplant (*S. melongena*), and pepper (*Capsicum annuum*). Plastid transformation has also been reported in other important crop species, including soybean (*Glycine max*, Fabaceae), lettuce (*Lactuca sativa*, Asteraceae), cauliflower (*Brassica oleracea* var. *botrytis*, Brassicaceae), cotton (*Gossypium* spp., Malvaceae), carrot (*Daucus carota*, Umbelliferae), bitter squash (*Momordica charantia*, Cucurbitaceae) and rice (*Oryza sativa*, Poaceae) (Table 1). To date, the plastids of over 20 flowering plants have been transformed [15] (Table 1). In addition to the crops mentioned above, recent successes in plastid transformation have been reported in the plant species bitter melon [16], and the medicinal plant sweet wormwood (*Artemisia annua*) [17] and licorice weed (*Scoparia dulcis*) [18,19] (Table 1).

**Table 1.** Species in which plastid transformation has been demonstrated.

1 Kanamycin; 2 Spectinomycin; 3 Streptomycin; 4 PEG-mediated transformations.

Based on these successful cases, plastid transformation should be applicable to many plant families, whether they are monocots or dicots. However, plastid transformation remains much more challenging than nuclear transformation and is not as widespread in plant research. Furthermore, Bock [7] raised the issue of reproducibility of plastid transformation, as the plastids of fewer than ten species have been demonstrably transformed in at least two independent reports since 1988, the year the first example of plastid transformation was published in the unicellular green alga Chlamydomonas (*Chlamydomonas reinhardtii*) [7,40]. Major critical points limiting current plastid transformation attempts

are 1) the method of DNA delivery, 2) homologous recombination efficiency and 3) methods for efficient selection and robust regeneration of transformants. In this review, we will address each of these issues and will highlight recent innovative technologies and strategies in plastid transformation. We hope to give readers a new perspective on the potential of plastid transformation with concrete examples, setting the stage for plastid transformation in more plant species in the future.


**2. Synthetic strategies for DNW growth**

#### **2.1. Plasma-assisted reactive-ion etching**

Reactive-ion etching (RIE) is an etching technology applied in micro- or nanofabrication, which may apply the dry etching than that of wet routes [25]. Wherein, plasma has been used to remove material deposited on substrates. A schematic diagram presented in **Figure 1** represents a typical RIE setup.

In general, the plasma is generated by an electromagnetic field under vacuum. Then, plasma produces the high-energy ions, which react with the surface of the sample to provide the desired nanostructures. However, its output also depends on the parameters such as power density, frequency, pressure, dc bias, gas composition, flow rate and so on. In 1997, Shiomi et al. informed the DNW growth by means of plasma-assisted RIE technique [26]. Thereafter, RIE technique has been widely applied for the growth of DNWs. However, later on, DNWs were grown up either with the support of mask or maskless processes.

#### *2.1.1. RIE with masks for DNW growth*

diamond nanowires (DNWs) are also known as a material of extremes, in which, its properties are exceptional in terms of band gap, electron affinity, chemical inertness, resistance to particle bombardment, hardness and thermal conductivity [12]. Moreover, upon tuning the n- or p-doping on DNWs, the diverse field emission, semiconductor and sensory applications can be attained. The diverse applications may be attributed to the lattice structures of those DNWs

benefits of DNW-based extensive research [15]. As a consequence of those defects or impurity channels, DNWs have the color centers, which enable their photonic applications such as single-photon emission [16]. Moreover, the toughness and wear resistance of DNWs may be

Attributed to the utilities of DNWs, numerous reports on their synthesis have been available so far. However, the synthesis of DNWs was claimed to be a low probability event in terms of reproducibility, which makes it as a thought-provoking task. Therefore, researchers tend to develop the suitable methods to grow the DNWs due to its potential benefits in ultraviolet (UV) light detectors and emitters [17, 18] radiation particle detectors [19], high-speed and high-power field effect transistors [20], field emission sources [21, 22], position-sensitive biochemical substrates [23] and room temperature-stabilized high-efficiency single-photon emitters [24]. So far, DNWs were grown from (1) Plasma-assisted reactive-ion etching process (RIE) with mask and maskless techniques; (2) chemical vapor deposition (CVD) techniques with diverse templates assistance, plasma enhancement, catalyst assistance, and so on; (3)

bon nanotubes (MWCNTs) with hydrogen plasma and from fullerenes). Similar to the above reported techniques on DNW growth, the development of hybrid graphene-DNWs (G-DNWs) also attracted the modern scientific research because of their diverse conductivity or semiconductor applications [12]. However, such nanowire (DNWs and G-DNWs) growth is still a challenging task; hence, an overview on its synthesis, structures and applications is required. In this chapter, we tend to present a brief report of the diamond nanowires, with discussions on DNW synthesis along with their structures, properties and applications. Wherein, the important synthetic pathways to grow the DNWs are pinpointed. Then, the comprehensive discussions on the structures and properties of the DNWs are derived from the available theoretical and experimental reports. Subsequently, the applications of those DNWs in diverse

Reactive-ion etching (RIE) is an etching technology applied in micro- or nanofabrication, which may apply the dry etching than that of wet routes [25]. Wherein, plasma has been used to remove material deposited on substrates. A schematic diagram presented in **Figure 1**

enhanced due to the hindering of dislocation movement by the impurities.

/sp3

ratio [13, 14]. However, the presence

) and impurity channels also enhances the

as well as the carbon-carbon bond or existence of sp2

of defects such as nitrogen vacancy center (NV−

18 Nanowires - Synthesis, Properties and Applications

sp2

fields are summarized.

**2. Synthetic strategies for DNW growth**

**2.1. Plasma-assisted reactive-ion etching**

represents a typical RIE setup.

The planar diamond films can be etched to obtain the DNWs. This also was attained with the support of several masks such as (1) metal nanoparticles mask, (2) oxide nanoparticles mask, and (3) diamond nanoparticles mask. However, the size and density of the developed DNWs depends on the nanoparticles that are used in masks, in which the size of those nanoparticles may lie in few nanometers.

(A) RIE with metal mask: After Shiomi's report [26] on DNW growth by using Al mask and oxygen plasma RIE, various columnar diamond nanowires with 300-nm length and 10-nm width have been constructed through etching CVD polycrystalline diamond films in O2 plasma [27, 28]. In this light, Liao et al. effectively developed the single-crystal diamond pillar like DNWs by Al-masked RIE technique [29]. However, Al-masked RIE method led to provide polycrystalline DNWs, hence having the disadvantages such as the presence of grain boundaries, impurities and large stresses in the films. Apart from Al-mask, other kinds of metals such as Mo, Ni, Fe and Au were also been utilized to develop doped or undoped DNWs [30–32]. Li and Hatta explored the effect of those metal masks, for the development of DNWs [33].

(B) RIE with oxide nanoparticles mask: Fujishima et al. successfully developed the DNWs through reactive-ion etching supported by oxygen plasma consisting of two-dimensional

**Figure 1.** A diagram of a common RIE setup. An RIE consists of two electrodes (1 and 4) that create an electric field (3) meant to accelerate ions (2) toward the surface of the samples (5) (https://en.wikipedia.org/wiki/Reactive-ion\_etching).

(2D) arrays of monodisperse solid SiO2 particles as masks [34]. Wherein, on the planar diamond surface, fine SiO2 particles are packed at high density [35] and oriented layers over a wide surface area by water evaporation and lateral capillary forces [36]. Then, reactive-ion etching (RIE) was carried out with oxygen plasma through the SiO2 arrays for 5–120 min in a plasma-etching machine with a radio frequency (RF) generator. Lastly, the SiO2 particles were detached from the diamond by HF-HNO3 treatment, which afford the DNW arrays. After this report, Hausmann et al. also elaborate the DNW synthesis by Al2 O3 mask [37], which found to be the most etch resistant. Hence, these flowable oxide masks are demonstrated to be a suitable etching mask for the construction of ordered arrays of DNWs.

**2.2. Chemical vapor deposition (CVD) for DNW synthesis**

*2.2.1. Template-assisted CVD methods for DNW growth*

B-doped DNW growth by Si nanowires template [46].

structures can be released from the alumina.

*2.2.2. Template-free CVD techniques for DNW growth*

Among the available effective methods for DNW synthesis, CVD technique is one of the promising processes utilized extensively [41]. This simple evaporation technique has been used to grow the elemental or oxide nanowires in an appropriate atmosphere. However, CVD process can be applied by means of template assistance or template-free ones as follows.

Diamond Nanowire Synthesis, Properties and Applications

http://dx.doi.org/10.5772/intechopen.78794

21

This is a convenient method to generate the 1D nanostructures and capable of producing nanostructures with exclusive structures, morphologies and properties [42–44]. Wherein, the template assists as a scaffold on which other materials with similar morphologies are produced. Moreover, they can be at nanoscale within mesoporous alumina or polycarbonate membranes. The following templates were applied so far to grow DNWs: (A) nanowires

(A) Nanowires templated CVD for DNW growth: This method has two steps such as (1) synthesis of various nanowire templates and (2) conformal coating of nanowires templates with nanodiamond, which may lead to form the DNWs by CVD technique. Firstly, May et al. explored the microdiamond coatings into tungsten wires through CVD to construct the DNWs [45]. Afterward, several researchers applied this technique by using silicon, carbide, tungsten, titanium and copper nanowires as a template for DNW synthesis. **Figure 3** illustrates such

(B) Anodic aluminum oxide (AAO) templated CVD: Masuda and coworkers illustrated the growth of polycrystalline DNWs and diamond-like carbon (DLC) nanotubes by means of anodic aluminum oxide templates in microwave plasma-assisted CVD [47], in which those alumina templates [48] were prepared by electrochemical anodization of an aluminum sheet. Alumina templates possessing the holey nanoporous membranes and nucleated with 50- and 5-nm diamond particles led to the formation of DNWs. In this process, the deposition of diamond through the alumina pores yields a continuous film and supports the formation of nanostructures. Finally, by immersing in concentrated phosphoric acid at 250°C, those nano-

More recently, template-free CVD methods for DNW synthesis attracted the scientific community. Those template-free CVD techniques are (A) microwave plasma-enhanced CVD

**Figure 3.** Fabrication of B-doped DNWs by Si-nanowires templates with CVD. Reproduced with permission from [46].

templated with CVD and (B) anodic aluminum oxide (AAO) templated CVD.

(C) RIE with diamond nanoparticles: As shown in **Figure 2**, by using diamond nanoparticles as a mask, Yang et al. described the vertically aligned DNW synthesis from boron-doped singlecrystalline CVD diamond films [38]. Initially, a microwave-assisted CVD technique is used to grow the boron-doped (P-type) diamonds with smooth surfaces by homoepitaxy on Ib diamond substrates. Next, diamond nanoparticles etching mask with well-defined size and quality is deposited. The size of those diamond nanoparticles (dissolved in water by ultrasonication to form a pseudostable suspension) lies between 8 and 10 nm. Thereafter, to seed diamond nanoparticles on the surface of a diamond substrate, the planar diamond film is immersed into the suspension and sonicated. After deposition, RIE in an O2 (97%)/CF4 (3%) gas mixture is applied to afford the vertically aligned DNWs, which has been utilized in DNA sensing [39].

#### *2.1.2. Maskless RIE for DNW growth*

The above mask methods have certain limitations and hence become unfavorable for largescale fabrication. Therefore, researchers tend to develop uncomplicated methods to remove some masks by additional chemical or physical processes that can grow the DNWs. In this way, Fujishima et al. described the synthesis of heavily B-doped DNWs (the boron doping level is 2.1 × 1021 B cm−3) through oxygen plasma without any additional mask [40]. Here, boron atoms on the diamond act as the mask during plasma etching, hence avoiding the deposition or removal of mask by additional steps. In detail, during the etching step, those boron oxide species are removed collectively with carbon atoms, and then they appear to redeposit near the tops of the DNWs, which may serve as an etching mask. This straightforward maskless method has been widely used for the synthesis of DNWs in recent times.

**Figure 2.** Schematic illumination of the fabrication of vertically aligned diamond nanowires using a top-down technology and using diamond nanoparticles as the etching mask. Reproduced with permission from [38].

## **2.2. Chemical vapor deposition (CVD) for DNW synthesis**

Among the available effective methods for DNW synthesis, CVD technique is one of the promising processes utilized extensively [41]. This simple evaporation technique has been used to grow the elemental or oxide nanowires in an appropriate atmosphere. However, CVD process can be applied by means of template assistance or template-free ones as follows.

#### *2.2.1. Template-assisted CVD methods for DNW growth*

(2D) arrays of monodisperse solid SiO2

20 Nanowires - Synthesis, Properties and Applications

detached from the diamond by HF-HNO3

*2.1.2. Maskless RIE for DNW growth*

mond surface, fine SiO2

particles as masks [34]. Wherein, on the planar dia-

treatment, which afford the DNW arrays. After this

O3

(97%)/CF4

arrays for 5–120 min in a

mask [37], which found

particles were

(3%) gas mixture

particles are packed at high density [35] and oriented layers over a

wide surface area by water evaporation and lateral capillary forces [36]. Then, reactive-ion

to be the most etch resistant. Hence, these flowable oxide masks are demonstrated to be a

(C) RIE with diamond nanoparticles: As shown in **Figure 2**, by using diamond nanoparticles as a mask, Yang et al. described the vertically aligned DNW synthesis from boron-doped singlecrystalline CVD diamond films [38]. Initially, a microwave-assisted CVD technique is used to grow the boron-doped (P-type) diamonds with smooth surfaces by homoepitaxy on Ib diamond substrates. Next, diamond nanoparticles etching mask with well-defined size and quality is deposited. The size of those diamond nanoparticles (dissolved in water by ultrasonication to form a pseudostable suspension) lies between 8 and 10 nm. Thereafter, to seed diamond nanoparticles on the surface of a diamond substrate, the planar diamond film is immersed

is applied to afford the vertically aligned DNWs, which has been utilized in DNA sensing [39].

The above mask methods have certain limitations and hence become unfavorable for largescale fabrication. Therefore, researchers tend to develop uncomplicated methods to remove some masks by additional chemical or physical processes that can grow the DNWs. In this way, Fujishima et al. described the synthesis of heavily B-doped DNWs (the boron doping level is 2.1 × 1021 B cm−3) through oxygen plasma without any additional mask [40]. Here, boron atoms on the diamond act as the mask during plasma etching, hence avoiding the deposition or removal of mask by additional steps. In detail, during the etching step, those boron oxide species are removed collectively with carbon atoms, and then they appear to redeposit near the tops of the DNWs, which may serve as an etching mask. This straightforward maskless method has been widely used for the synthesis of DNWs in recent times.

**Figure 2.** Schematic illumination of the fabrication of vertically aligned diamond nanowires using a top-down technology

and using diamond nanoparticles as the etching mask. Reproduced with permission from [38].

etching (RIE) was carried out with oxygen plasma through the SiO2

report, Hausmann et al. also elaborate the DNW synthesis by Al2

into the suspension and sonicated. After deposition, RIE in an O2

suitable etching mask for the construction of ordered arrays of DNWs.

plasma-etching machine with a radio frequency (RF) generator. Lastly, the SiO2

This is a convenient method to generate the 1D nanostructures and capable of producing nanostructures with exclusive structures, morphologies and properties [42–44]. Wherein, the template assists as a scaffold on which other materials with similar morphologies are produced. Moreover, they can be at nanoscale within mesoporous alumina or polycarbonate membranes. The following templates were applied so far to grow DNWs: (A) nanowires templated with CVD and (B) anodic aluminum oxide (AAO) templated CVD.

(A) Nanowires templated CVD for DNW growth: This method has two steps such as (1) synthesis of various nanowire templates and (2) conformal coating of nanowires templates with nanodiamond, which may lead to form the DNWs by CVD technique. Firstly, May et al. explored the microdiamond coatings into tungsten wires through CVD to construct the DNWs [45]. Afterward, several researchers applied this technique by using silicon, carbide, tungsten, titanium and copper nanowires as a template for DNW synthesis. **Figure 3** illustrates such B-doped DNW growth by Si nanowires template [46].

(B) Anodic aluminum oxide (AAO) templated CVD: Masuda and coworkers illustrated the growth of polycrystalline DNWs and diamond-like carbon (DLC) nanotubes by means of anodic aluminum oxide templates in microwave plasma-assisted CVD [47], in which those alumina templates [48] were prepared by electrochemical anodization of an aluminum sheet. Alumina templates possessing the holey nanoporous membranes and nucleated with 50- and 5-nm diamond particles led to the formation of DNWs. In this process, the deposition of diamond through the alumina pores yields a continuous film and supports the formation of nanostructures. Finally, by immersing in concentrated phosphoric acid at 250°C, those nanostructures can be released from the alumina.

#### *2.2.2. Template-free CVD techniques for DNW growth*

More recently, template-free CVD methods for DNW synthesis attracted the scientific community. Those template-free CVD techniques are (A) microwave plasma-enhanced CVD

(MPCVD), (B) hot cathode direct current plasma CVD (HCDC-PCVD), and (C) catalystassisted atmospheric pressure CVD. Detailed information of the above CVD methods is presented subsequently.

(A) Microwave plasma-enhanced CVD (MPCVD): At first, Valsov et al. presented the synthesis of hybrid graphite-diamond nanowires (G-DNWs) over an ultrananocrystalline diamond (UNCD) film by using MPCVD technique [49]. Afterward, Shang and coworkers described the development of ultrathin diamond nanorods (UDNRs) by this method [50]. However, the incorporation of N2 becomes essential as demonstrated by recent reports [51, 52], wherein the incorporation of N2 enhances the electrical conductivity through tuning the sp2 /sp3 carbon ratio. Hypothetically, the introduction of nitrogen into plasma may motivate the formation of molecular CN species, thereby generating favorable conditions for an increase in the grain size as well as the formation of ID diamond nanostructures. In general, DNWs synthesized through MPCVD technique show good electrochemical properties due to the rise of sp2 content, new C-N bonds at the grains and an escalation in the electrical conductivity at the grain boundaries [53].

(B) Hot cathode direct current plasma CVD (HCDC-PCVD): This is an innovative technique for the deposition of nano- and micro-crystalline diamond films with uniformity over a large area and with a high growth rate. Here, the cathode made up of a tantalum disc linked to a water-cooled cylindrical copper block, water-cooled copper block anode and a nonpulsedtype dc power source is used. From this technique, Zeng et al. explored the formation of DNRs along with (111) diamond microcrystals and (100) diamond microcrystals on Si substrates [54].

(C) Catalyst-assisted atmospheric pressure CVD: Apart from the previously mentioned hightemperature methods assisted by plasma or energy radiation, the production of long singlecrystalline DNWs by conventional thermal CVD methods has become essential because of their potential benefits. In this light, Hsu et al. described the growth of DNWs by means of CVD without plasma or energy sources and at atmospheric pressure [55]. Here, methane and hydrogen were flowed into the Fe catalyst solution, which was dispersed on an Si substrate at 900° C. Then, pure hydrogen was run through the quartz tube chamber (at 200 sccm as rate) without pumping the residual methane. Subsequently, the temperature lowers down to an ambient condition at a rate of 1.2° C min−1 for 12 h. This method will produce the uniform long and thin DNWs with a diameter of 60–90 nm. Importantly, in this process, hydrogen plays a vital role in the formation of DNWs via sp- and sp2 -hybridized bonds transformation into sp3 -hybridized atoms [56]. **Figure 4** demonstrates the possible vapor-liquid-solid (VLS) mechanism for the growth of DNWs by this method [57]. This technique supports the exceptional utilization CVD with the support of transition metal catalyst such as Fe and also envisioned the applicability of CVD at atmospheric pressure.

however, it is still a challenging task. The following are few examples of transformation of

the diamond phase could be more stable but the capillary pressure rapidly decreases with diameter, leaving the shell more stable in the graphitic phase. (d) The grain growth, boundary healing and structural reorganization take place in

the slow-cooling period in the presence of a pure hydrogen flow. Reproduced with permission from [57].

**Figure 4.** Schematic diagram showing a possible formation process of diamond nanowires: (a) catalytic particles are formed from the evaporated or deposited thin film on the substrate as the temperature rises; (b) carbon-containing radicals reach the surface of the catalysts, leading to the growth of either a diamond stud or a graphitic tube via the VLS mechanism. The size is determined by the catalyst. (c) Hydrogen assists in the growth process by either preferentially

by hydrogen plasma post-treatment of multiwalled carbon nanotubes, (B) DNW growth from

(A) DNW synthesis by hydrogen plasma post-treatment of multiwalled carbon nanotubes (MWCNTs): In 2005, Sun and coworkers presented this simple method for the growth of DNWs from carbon nanotubes (CNTs) via hydrogen plasma post treatment [58]. Impressively, upon extended hydrogen plasma treatment, the DNWs with the diameters of 4–8 nm and with the lengths up to several hundreds were obtained. This work also revealed the TEM of single-crystal DNWs from the MWCNTs after treatment in hydrogen plasma at 1000 K for 20 h. The author proposed the mechanism as clustering, crystallization, growth and faceting, which is similar to the report by Singh et al. [59]. It is also established that the presence of amorphous carbon sheath over diamond nanoparticles and DNWs is responsible for this kind of transformation. (B) DNW growth from fullerenes: In 2005, Dubrovinskaia and coworkers synthesized a bulk sample of nanocrystalline cubic diamond from fullerene C60, which have the crystallite sizes of 5–12 nm and a hardlike single-crystal diamond [60]. **Figure 5** represents the TEM of those ADNRs. These nanocrystalline diamonds seem to be highly stable at an elevated temperature

carbon, which led to the formation of DNWs: (A) DNW synthesis

bonds. With the higher capillary pressure at smaller diameters,

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sp2

etching sp. and sp2

graphite carbon to sp3

fullerenes and (C) DNWs from diamondoids.

bonds or transforming them into sp3

#### **2.3. DNW growth from sp2 carbon and sp3 diamondoids**

Attributed to the importance, the conversion of sp2 graphite carbon to sp3 diamond crystals remains to be a challenging task over many years for which high pressures and high temperatures are required. Recently, researchers developed few methods for such transformation;

(MPCVD), (B) hot cathode direct current plasma CVD (HCDC-PCVD), and (C) catalystassisted atmospheric pressure CVD. Detailed information of the above CVD methods is pre-

(A) Microwave plasma-enhanced CVD (MPCVD): At first, Valsov et al. presented the synthesis of hybrid graphite-diamond nanowires (G-DNWs) over an ultrananocrystalline diamond (UNCD) film by using MPCVD technique [49]. Afterward, Shang and coworkers described the development of ultrathin diamond nanorods (UDNRs) by this method [50]. However, the

ratio. Hypothetically, the introduction of nitrogen into plasma may motivate the formation of molecular CN species, thereby generating favorable conditions for an increase in the grain size as well as the formation of ID diamond nanostructures. In general, DNWs synthesized through MPCVD technique show good electrochemical properties due to the rise of sp2

tent, new C-N bonds at the grains and an escalation in the electrical conductivity at the grain

(B) Hot cathode direct current plasma CVD (HCDC-PCVD): This is an innovative technique for the deposition of nano- and micro-crystalline diamond films with uniformity over a large area and with a high growth rate. Here, the cathode made up of a tantalum disc linked to a water-cooled cylindrical copper block, water-cooled copper block anode and a nonpulsedtype dc power source is used. From this technique, Zeng et al. explored the formation of DNRs along with (111) diamond microcrystals and (100) diamond microcrystals on Si sub-

(C) Catalyst-assisted atmospheric pressure CVD: Apart from the previously mentioned hightemperature methods assisted by plasma or energy radiation, the production of long singlecrystalline DNWs by conventional thermal CVD methods has become essential because of their potential benefits. In this light, Hsu et al. described the growth of DNWs by means of CVD without plasma or energy sources and at atmospheric pressure [55]. Here, methane and hydrogen were flowed into the Fe catalyst solution, which was dispersed on an Si substrate

C. Then, pure hydrogen was run through the quartz tube chamber (at 200 sccm as rate) without pumping the residual methane. Subsequently, the temperature lowers down to an

long and thin DNWs with a diameter of 60–90 nm. Importantly, in this process, hydrogen

(VLS) mechanism for the growth of DNWs by this method [57]. This technique supports the exceptional utilization CVD with the support of transition metal catalyst such as Fe and also

remains to be a challenging task over many years for which high pressures and high temperatures are required. Recently, researchers developed few methods for such transformation;

 **diamondoids**

C min−1 for 12 h. This method will produce the uniform

graphite carbon to sp3

diamond crystals

becomes essential as demonstrated by recent reports [51, 52], wherein the

/sp3

carbon

con-

enhances the electrical conductivity through tuning the sp2

sented subsequently.

22 Nanowires - Synthesis, Properties and Applications

incorporation of N2

incorporation of N2

boundaries [53].

strates [54].

at 900°

tion into sp3

ambient condition at a rate of 1.2°

**2.3. DNW growth from sp2**

plays a vital role in the formation of DNWs via sp- and sp2

envisioned the applicability of CVD at atmospheric pressure.

Attributed to the importance, the conversion of sp2

 **carbon and sp3**

**Figure 4.** Schematic diagram showing a possible formation process of diamond nanowires: (a) catalytic particles are formed from the evaporated or deposited thin film on the substrate as the temperature rises; (b) carbon-containing radicals reach the surface of the catalysts, leading to the growth of either a diamond stud or a graphitic tube via the VLS mechanism. The size is determined by the catalyst. (c) Hydrogen assists in the growth process by either preferentially etching sp. and sp2 bonds or transforming them into sp3 bonds. With the higher capillary pressure at smaller diameters, the diamond phase could be more stable but the capillary pressure rapidly decreases with diameter, leaving the shell more stable in the graphitic phase. (d) The grain growth, boundary healing and structural reorganization take place in the slow-cooling period in the presence of a pure hydrogen flow. Reproduced with permission from [57].

however, it is still a challenging task. The following are few examples of transformation of sp2 graphite carbon to sp3 carbon, which led to the formation of DNWs: (A) DNW synthesis by hydrogen plasma post-treatment of multiwalled carbon nanotubes, (B) DNW growth from fullerenes and (C) DNWs from diamondoids.

(A) DNW synthesis by hydrogen plasma post-treatment of multiwalled carbon nanotubes (MWCNTs): In 2005, Sun and coworkers presented this simple method for the growth of DNWs from carbon nanotubes (CNTs) via hydrogen plasma post treatment [58]. Impressively, upon extended hydrogen plasma treatment, the DNWs with the diameters of 4–8 nm and with the lengths up to several hundreds were obtained. This work also revealed the TEM of single-crystal DNWs from the MWCNTs after treatment in hydrogen plasma at 1000 K for 20 h. The author proposed the mechanism as clustering, crystallization, growth and faceting, which is similar to the report by Singh et al. [59]. It is also established that the presence of amorphous carbon sheath over diamond nanoparticles and DNWs is responsible for this kind of transformation.

(B) DNW growth from fullerenes: In 2005, Dubrovinskaia and coworkers synthesized a bulk sample of nanocrystalline cubic diamond from fullerene C60, which have the crystallite sizes of 5–12 nm and a hardlike single-crystal diamond [60]. **Figure 5** represents the TEM of those ADNRs. These nanocrystalline diamonds seem to be highly stable at an elevated temperature

**Figure 5.** (a) Bright-field TEM image of a nanocrystalline aggregate with needle-shaped, elongated crystals diamond nanorods. The crystals can be longer than 1 μm, whereby the needle width is only about 20 nm or less; (b) bright-field image shows a close-up of the elongated crystals. The long edges of the crystals are parallel to the (111) plane, and the needle axes are approximately parallel (211)\*. Reproduced with permission from [60].

In this way, with respect to Berman et al. report on metal-induced graphitization of diamond particles [64], metal ions induced G-DNWs formation is also seem to be highly feasible. However, the reproducibility and percentage formation of G-DNWs by this path is still a challenging task. Currently, our group is working on this research to grow the G-DNWs

representing amorphous graphite along with

region representing less perfect graphite layer

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In order to establish the diverse applications of DNWs, the structure and properties should be elucidated. The properties such as structural stability, mechanical properties, density and compressibility, photon optical mode and electronic structure, thermal conductivity and electrochemical properties play vital role in their applications. Hence, researchers described the experimental and theoretical investigations on the structure and properties of DNWs as

From theoretical investigations, it has been found that dehydrogenated C(111) octahedral nanodiamond surfaces are structurally unstable. However, cuboctahedral structures of nanodiamond may increase the C(100) surface area and become more stable, which also reduce the surface graphitization. In this light, Barnard et al. investigated three kinds of DNWs including dodecahedral, cubic and cylindrical nanowires and found that nanocrystalline diamonds are structurally stable at one dimension [65]. Moreover, they also demonstrate that stability depends on the surface morphology and crystallographic direction of the principal axis of DNWs. In a similar fashion, Tanskanen and coworkers established the structures of polyicosahedral DNWs derived from diamondoids, C20H20, C20@C80H60, and C20@C80@C180H120. For which they have summarized the HOMO-LUMO gaps, and band gaps via B3LYP calculations [66]. Wherein, the C20@C80@C180H120 structures are energetically favored and the DNWs at 110 direction have the lowest strain energies leading to more stability. This has been experimentally

with good reproducibility.

**3.1. Structural stability of DNWs**

follows.

**3. Structures and properties of DNWs**

**Figure 6.** (a) HR-TEM image of G-DNWs, (b) FT pattern of selected area **a1**

along with defects or impurity channels. Reproduced with permission from [63].

diamond (111) diffraction pattern and (c) high magnification image of **a1**

and an ambient pressure. In the meantime, they developed the aggregated diamond nanorods (ADNRs) from C60 by multi-anvil apparatus [61]. Those ADNRs have the diameter of 5–20 nm and have the length of more than 1 μm.

(C) DNWs from diamondoids: Similar to carbon nanotubes and fullerenes, diamondoids may also lead to the formation of DNWs. The 1D diamondoid aggregates confined in CNTs directed to form the DNWs via 'face-fused' reaction. However, these transformations of adamantane into DNWs seem to be energetically not feasible. Contrarily, Zhang et al. explored the theoretical and experimental proof for these fusion reactions by diamantane-4,9-dicarboxylic acid transformation to 1D diamond nanowires inside CNTs [62]. In which, the fusion of diamantane-4,9-dicarboxylic acid under the confinement of CNTs yields the DNWs.

#### **2.4. Wet chemical route to synthesis DNWs**

Attributed to the applications of DNWs, numerous efforts have been made by the researchers to synthesize them. Among them, wet chemical route seems to be impressive with respect to cost-effectiveness than that of RIE and CVD techniques. But it is also essential to make them with reproducibility and uniformity. To this footpath, recently, our group report the pH-induced electrostatic self-assembly of novel cysteamine functionalized diamond nanoparticles (**ND-Cys**) to evidence hybrid G-DNW growth [63]. Those G-DNWs are highly stable in respective pH buffers, but if more amount of DI-water is added, the longer nanowires (initially at ~100 μm) break into small wires/rods (few microns). At pH 6, the width of G-DNWs ranges between 20 and 800 nm and the length lies between 200 nm and hundreds of microns with respect to dispersion concentration. Wherein, the DNW formation was initiated through electrostatic forces within the partially graphitized ND-Cys particles. Next, those partially graphitized ND-Cys particles and defects/impurity channels were further promoted to form the graphene shells on the surface of DNPs and sandwiched between the diamond cores. These G-DNWs show exceptional conductivity due to the presence of defects and impurity channels. **Figure 6** illustrates the TEM image of those nanowires with defect or impurity channels.

**Figure 6.** (a) HR-TEM image of G-DNWs, (b) FT pattern of selected area **a1** representing amorphous graphite along with diamond (111) diffraction pattern and (c) high magnification image of **a1** region representing less perfect graphite layer along with defects or impurity channels. Reproduced with permission from [63].

In this way, with respect to Berman et al. report on metal-induced graphitization of diamond particles [64], metal ions induced G-DNWs formation is also seem to be highly feasible. However, the reproducibility and percentage formation of G-DNWs by this path is still a challenging task. Currently, our group is working on this research to grow the G-DNWs with good reproducibility.