**Abstract**

This chapter covers recent advances in the development of nanostructurebased material technologies to benefit next-generation electro-optical (EO) and infrared (IR) sensor and imager applications. Nanostructured materials can now be integrated into a variety of technological platforms, offering novel optoelectrical properties that greatly enhance device performance in many practical applications. Use of novel carbon nanotube (CNT) based materials has enabled new approaches for applying nanostructure design methodologies that can offer enhanced performance for low-cost bolometers for IR detection and imaging applications. We will discuss the development of carbon nanostructure based infrared detectors and arrays, including concepts that will provide high performance, high frame rate, and uncooled microbolometers for mid-wave infrared (MWIR) and long-wave infrared (LWIR) band detection. In addition, nanostructured antireflection (AR) coatings are being developed that significantly enhance transmission over a broad spectrum, providing substantial improvements in device performance compared to conventional thin film AR coatings. These nanostructured AR coatings have been demonstrated over visible to LWIR spectral bands on various substrates. In this chapter, we discuss both theoretical and measured results of these diverse nanostructure technologies to advance sensing performance over a wide range of spectral bands for defense, space, and commercial applications.

**Keywords:** sensors, detectors, infrared, electro-optical, carbon nanotubes nanostructures, antireflection coatings

#### **1. Introduction**

Nanostructures come in many different physical forms having varied and diverse optoelectronic properties that enable them to benefit many different applications, of which electro-optic/infrared (EO/IR) detectors and systems may be considered in the forefront. Rapid advances in these technologies that have recently taken place are enabling the development of more capable sensing and imaging systems and subsystems with notable improvements in performance. Such nanostructure-based devices and systems operating from the visible to long-wave infrared (LWIR) portions of the electromagnetic spectrum have been and continue to be developed for use in a variety of defense and commercial applications [1, 2].

Detectors and imaging arrays operating over the visible spectrum (~400– 700 nm) are mostly Si-based, consisting of charge-coupled device (CCD) imagers and complementary metal oxide semiconductor (CMOS) technologies. For detection of infrared (IR) wavelengths invisible to the human eye, which span the range of approximately 1.0 μm in the near-infrared (NIR) to beyond 30 μm, other materials and technologies are required [3].

The IR spectrum is categorized based primarily on the spectral locations of atmospheric transmission windows (**Figure 1**). IR radiation falling within these bands can generally be transmitted and/or received with relatively low attenuation. The NIR and short-wave infrared (SWIR) bands (~0.8–2.5 μm) like the visible mostly involve reflected light, for which materials including InGaAs, InP, and Ge are used for detection and imaging. For the mid-wave infrared (MWIR: 3–5 μm) and LWIR (8–14 μm) wavebands, which conversely involve detection of objects emitting thermal radiation, HgCdTe and InSb based focal plane arrays (FPAs), strained-layer superlattice (SLS) structures, and microbolometers (e.g., Si-MEMS devices) have been and are being utilized [4].

Uncooled microbolometers used as detectors in thermal cameras have traditionally been based on silicon as well as vanadium oxide material deposited on silicon. However, the use of novel carbon nanotube-based detector elements in IR microbolometer pixels is attractive due to the potential for high electron mobilities and high temperature sensitivity [5]. Carbon nanotubes (CNTs) basically comprise hexagonal sheets of graphene, a material with enhanced electrical capabilities, rolled into cylinders that exhibit exceptional conductivity and strength [6].

In following sections of this chapter, we discuss integration of CNTs in detector elements for next-generation high performance, high frame rate, and uncooled bolometer arrays that can provide enhanced sensitivity for MWIR and LWIR band detection and imaging [7, 8].

Sensors and detector arrays generally comprise semiconductor materials characterized by high refractive indexes, which results in significant optical reflection from the top surface that reduces light throughput into and thus light absorbed by the detector. To address this, nanostructured antireflection (AR) coatings have been developed to reduce unwanted reflections and increase the transmission of light from visible to LWIR wavelengths, thereby improving detector performance [9, 10].

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**Figure 2.**

*Diagram showing multiple mechanisms for inducing a band gap in graphene [5, 11].*

*Nanostructure Technology for EO/IR Detector Applications*

**2.1 Properties and applications of carbon nanotubes**

We likewise discuss in this chapter the growth processes, performance, and device applications of these advanced nanostructured AR coatings capable of minimizing reflection losses over a wide range of wavelengths and incident angles.

Graphene is a two-dimensional analogue of carbon-based graphite material

There are multiple pathways for creating graphene sheets, which include: exfoliation, unzipping through etching, growth from sublimation, and epitaxial growth from a catalyst layer [11]. The absence of a band gap in graphene limits voltage and power gains that may be achieved through operation of a device in the saturation regime. To overcome this, several doping strategies as shown in **Figure 2** have been proposed and tested, including: electrostatic doping, chemical doping, and stress or geometry restricted doping by breaking the graphene periodicity [5, 11]. Since it can be doped electrostatically, graphene provides a useful solution for certain electronic applications such the channels in field effect transistors (FETs) [12].

Carbon nanotubes (CNTs), which like graphene are allotropes of carbon, may be considered formed of hexagonal sheets of graphene rolled into cylinders (**Figure 3**). CNTs exhibit bandgaps in the range of zero to ~2 eV, and demonstrate metallic or semiconducting behavior depending mainly on their diameter and orientation of the hexagonal graphene structure along the axis of the tube (armchair, zigzag, or

/Vs, derived from the bonding characteristics of the carbon sheets [5].

that has exceptional electrical characteristics, e.g., mobilities up to 15,000–

**2. Carbon nanotube-based microbolometers for IR imaging**

*DOI: http://dx.doi.org/10.5772/intechopen.85741*

200,000 cm2

**Figure 1.** *IR spectral band atmospheric windows [3].*

*Nanorods and Nanocomposites*

materials and technologies are required [3].

devices) have been and are being utilized [4].

strength [6].

detection and imaging [7, 8].

detector performance [9, 10].

*IR spectral band atmospheric windows [3].*

Detectors and imaging arrays operating over the visible spectrum (~400– 700 nm) are mostly Si-based, consisting of charge-coupled device (CCD) imagers and complementary metal oxide semiconductor (CMOS) technologies. For detection of infrared (IR) wavelengths invisible to the human eye, which span the range of approximately 1.0 μm in the near-infrared (NIR) to beyond 30 μm, other

The IR spectrum is categorized based primarily on the spectral locations of atmospheric transmission windows (**Figure 1**). IR radiation falling within these bands can generally be transmitted and/or received with relatively low attenuation. The NIR and short-wave infrared (SWIR) bands (~0.8–2.5 μm) like the visible mostly involve reflected light, for which materials including InGaAs, InP, and Ge are used for detection and imaging. For the mid-wave infrared (MWIR: 3–5 μm) and LWIR (8–14 μm) wavebands, which conversely involve detection of objects emitting thermal radiation, HgCdTe and InSb based focal plane arrays (FPAs), strained-layer superlattice (SLS) structures, and microbolometers (e.g., Si-MEMS

Uncooled microbolometers used as detectors in thermal cameras have traditionally been based on silicon as well as vanadium oxide material deposited on silicon. However, the use of novel carbon nanotube-based detector elements in IR microbolometer pixels is attractive due to the potential for high electron mobilities and high temperature sensitivity [5]. Carbon nanotubes (CNTs) basically comprise hexagonal sheets of graphene, a material with enhanced electrical capabilities, rolled into cylinders that exhibit exceptional conductivity and

In following sections of this chapter, we discuss integration of CNTs in detector elements for next-generation high performance, high frame rate, and uncooled bolometer arrays that can provide enhanced sensitivity for MWIR and LWIR band

Sensors and detector arrays generally comprise semiconductor materials characterized by high refractive indexes, which results in significant optical reflection from the top surface that reduces light throughput into and thus light absorbed by the detector. To address this, nanostructured antireflection (AR) coatings have been developed to reduce unwanted reflections and increase the transmission of light from visible to LWIR wavelengths, thereby improving

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**Figure 1.**

We likewise discuss in this chapter the growth processes, performance, and device applications of these advanced nanostructured AR coatings capable of minimizing reflection losses over a wide range of wavelengths and incident angles.
