Fahlman B.D. Materials Chemistry

Подождите немного. Документ загружается.

Rather than having to use a mold for nanocontact printing, one may also directly

write features onto an appropriate substrate using a molecular ink, via the ultra-ﬁne

tip of an atomic force microscope

[79]

(Figure 4.75). The use of such a “nanofountain

pen” is known as dip-pen nanolithography (DPN), ﬁrst demonstrated by Mirkin and

Figure 4.71. Schematic illustration of the particle/pattern replication in nonwetting templates (PRINT)

process and traditional embossing processes. Shown are: (a) silicon master template, (b) mold release

from master template, and (c) mold ﬁlling via capillary ﬁll with counter-sheet having a higher surface

energy than the PFPE mold. Depending on the exact nature of the liquid to be molded and the details of the

process, (d) one can ﬁll the cavities only and not wet the land area around the cavities or (d

) one can

ﬁll the cavities and have a thin layer of liquid on the land area around the cavities. The thickness of the

layer of connecting ﬂash layer liquid is determined from the principles associated with free meniscus

coating processes with the resulting (e, e

) pattern transfer to substrate, (f, f

) mold release from array of

isolated features, and (g) dissolution of the harvesting ﬁlm to yield free particles. As an alternative to

PRINT, one can use PFPEs using traditional embossing processes where pressure and heat are applied

(h, i) to form an embossed ﬁlm (j) after the mold is removed. Reproduced with permission from Acc.

318 4 Semiconductors

coworkers in the late 1990s.

[80]

Though the earliest examples of DPN featured

alkylthiols as the ink onto Au surfaces, there are now an increasingly large number

of other ink/substrate combinations that have been reported for DPN (Table 4.5).

[81]

A general beneﬁt of DPN over other soft lithographic techniques is the ability to

pattern nanostructures (including biological materials) by a single step without

cross-contamination, since the desired chemistry occurs only in a speciﬁcally

deﬁned location of the substrate.

The mechanism of ink transport from the AFM tip to substrate is currently an item

of controversy. Recent models suggest that a meniscus forms between the tip and

substrate, which aids in ink transport. As a result, the transport rate is found to

increase concomitantly with the ambient humidity – but only for inks that are

soluble in water. In general, the rate of ink transport is found to decrease signiﬁ-

cantly with increasing contact time, due to the changing surface energy of the

substrate. As you might imagine, there are many factors that govern ink transport

and the ﬁnal resolution of the printed nanostru cture – tip shape, ink composition/

concentration, substrate surface properties, and ambient conditions. It is clear that

Figure 4.72. Schematic of soft lithographic procedures for (a) replica molding (REM), (b) microtransfer

molding (mTM), (c) micromolding in capillaries (MIMIC), and (d) solvent-assisted micromolding

Wiley-VCH.

4.2. Silicon-Based Applications 319

Figure 4.73. PRINT particles varying in size, shape, surface chemistry, and deformability. The particle

composition for all of these particles was approximately the same and included PEG (bulk of the matrix), a

cross-linker, and a linker group for conjugation of stabilizing groups (such as PEG) or targeting ligands

320 4 Semiconductors

the current 15-nm reso lution of DPN may only be improved once a full picture of ink

transport is fully understood.

4.3. LIGHT-EMITTING DIODES: THERE IS LIFE OUTSIDE

OF SIL ICON!

Thus far, we have considered the structure and applications of Si – the most heavily

employed semiconductor. It should be noted that gallium arsenide (GaAs) is also

widely used for FET applications; since GaAs does not form a natural protective

Figure 4.73. Continued (such as peptides, antibodies, etc.): (a) scanning electron micrograph (SEM) of

cube-shaped particles with a cube side length of 5 m m; (b) SEM of cylindrical nanoparticles having

diameter) 110 nm and height) 35 nm; (c) SEM of cylindrical nanoparticles having diameter) 200 nm and

height) 200 nm; (d) SEM of rod-like PRINT particles having diameter) 100 nm and height) 300 nm;

(e) SEM of 3 mm “hex nut” particles; (f) cylindrical PRINT particles containing a covalently attached red

ﬂuorophore that have been functionalized on one face with a generic linker group (green ﬂuorophore) that

will allow the conjugation of targeting peptides, antibodies, and aptamers region-speciﬁcally onto the

particle probes; (g, h) particles for mechanobiology studies having approximately the same dimensions as

red blood cells (cylinders with a diameter) 7 mm and a height of 1.7 mm made from (g) a nondeformable,

highly cross-linked hydrogel and (h) lightly cross-linked, deformable hydrogel. Reproduced with

Figure 4.74. Illustration of the PRINT process compared to traditional imprint lithography in which the

afﬁnity of the liquid precursor for the surface results in a scum layer. In PRINT, the nonwetting nature

of ﬂuorinated materials and surfaces (shown in green) conﬁnes the liquid precursor inside the features

of the mold, allowing for the generation of isolated particles. Reproduced with permission from J. Am.

4.3. Light-Emitting Diodes: There is Life Outside of Silicon! 321

layer analogous to SiO

onto Si, CVD is used to grow ﬁlms such as GaS for surface

passivation. Many other direct bandgap semiconductors such as II–VI (e.g.,ZnS,

ZnSe) and III–V (e.g., GaN, GaP) are widely used for optoelectronic (light emission)

and photonic (light detection) applications. The most important applications for non-

Si semiconductors are light-emitting diodes (LEDs) and solid-state lasers. Unlike Si-

based devices, the bandgap of compound semiconductors may be signiﬁcantly

altered by varying the stoichiometry of the composite elements.

Figure 4.75. Illustration of dip-pen nanolithography, used to write nanoscale features of CdS on mica and

SiO

substrates. Reproduced with permission from Ding, L.; Li, Y.; Chu, H.; Li, X.; Liu, J. J. Phys. Chem.

Table 4.5. Summary of the Ink-Substrate Combinations Used to Date for DPN

[82]

Molecular ink Substrate

Alkylthiols (e.g., ODT

and MHA

)Au

Ferrocenylthiols Au

Silazanes SiO

, GaAs

Proteins Au, SiO

Conjugated polymers SiO

DNA Au, SiO

Fluorescent dyes SiO

Sols SiO

Metal salts Si, Ge

Colloidal particles SiO

Alkynes Si

Alkoxysilanes SiO

ROMP materials SiO

Thioacetamide/cadmium acetate

SiO

, mica

1-Octadecanethiol.

16-Mercaptohexadecanoic acid, or thiohexadecanoic acid.

Ding, L.; Li, Y.; Chu, H.; Li, X.; Liu, J. J. Phys. Chem. B 2005, 109, 22337.

322 4 Semiconductors

A light-emitting diode is a p–n junction device that is comprised of a direct

bandgap semiconductor(s). The recombination of an electron-hole pair (EHP)

results in photon emission, with energy equivalent to the bandgap, E

(Figure 4.76).

The LED may be structured as a simple multilayer (Figure 4.77a), or as a hetero-

junction that is comprised of two different bandgap semiconductors (Figure 4.77b).

The latter is preferred for high-intensity LED applications, since the emission is

conﬁned to certain regions of the device.

At present, the applications for LEDs have been centere d on commercial elec-

tronic displays such as clock radios, microwave ovens, watche s, etc. However, the

availability of LEDs in a spectrum of colors has opened the ﬂoodgates for new

applications. The greatest breakthrough was realized in the 1990s with the discovery

of wide bandgap blue LEDs, making it possible to create any color of light

(Table 4.6). Approximately 10–15% of the trafﬁc lights in the United States have

now been replaced with LED-based lamps. New automobiles also utilize this

lighting for ultrabright brake and turn-signal lighting. The higher initial cost of the

LEDs is quickly recovered due to their greater efﬁciency in converting electrical

current to light emission relative to incandescent lighting. That is, whereas LEDs

consume 20–1,000 mW, an incandescent bulb of similar brightness consumes ca.

50–150 W. Indeed, once an inexpensive white LED becomes commercially avail-

able, our society will forever change as we shift from our longstanding reliance on

inefﬁcient and short-lived incandescent bulbs. The light emission from LEDs is

classiﬁed as a type of luminescence, which is different from incandescence – the

generation of light from a material as a result of its high temperature. Since LEDs

glow as a result of an electrical current, this emission is referred to as electrolumi-

nescence. Two other common types of luminescence include chemoluminescence

Figure 4.76. Band diagram of a p–n junction with a) no bias voltage, and b) with forward bias, V,

resulting in photon emission. Reproduced with permission from Kasap, S. O. Principles of Electronic

Companies.

4.3. Light-Emitting Diodes: There is Life Outside of Silicon! 323

(induced by a chemical reaction(s); e.g., “glow sticks”), and photoluminescence

(induced through photon excitation; e.g., vaseline glass discussed in Chapter 2).

If the emission is prolonged, lasting long after the stimulation source is removed,

it is known as phosphorescence; otherwise, the short-lived process is termed

ﬂuorescence.

Figure 4.77. Light-emitting diode (LED) structures. Shown are (a) a simple multilayer structure of

epitaxial p- and n-type layers, and (b) a double heterostructure with accompanying band diagram,

illustrating light emission from the p-type region due to conﬁnement between wide-bandgap

surrounding layers.

Table 4.6. Comparison of the Observed Colors of LEDs

Observed color Wavelength of

emission (nm)

Semiconductor

(Infrared) 880 GaAlAs/GaAs

Red 660 GaAlAs/GaAlAs

Red 633 AlGaInP

Orange 612 AlGaInP

Orange 605 GaAsP/GaP

Yellow 585 GaAsP/GaP

Green 555 GaP

Blue 470 GaN/SiC

Ultraviolet 395 InGaN/SiC

White – InGaN/SiC

324 4 Semiconductors

As we saw earlier in this chapter, the wavelength (and color) of light emitted by a

direct bandgap material through electron-hole recombination is inﬂuenced by its

bandgap. In order to change the wavelength of emitted radiation, the bandgap

of the semiconducting material utilized to fabricate the LED must be changed.

For instance, gallium arsenide has a bandgap of 1.35 eV (Table 4.7), and emits in

the infrared (ca . 900 nm). In order to decrease the wavelength of emission into the

visible red region (ca. 700 nm), the bandgap must be increased to ca. 1.9 eV. This

may be achieved by mixing GaAs with a material with a larger bandgap, such as GaP

¼ 2.35 eV). Hence, LEDs of the chemical composition GaAs

1x

may be used

to produce bandgaps from 1.4 to 2.3 eV (and varying colors), through adjustment of

the As:P ratio.

The bandgap and concomitant wavelength of light that is emitted from LEDs is

related to the bond strength between atoms in the lattice. For these compounds, as

the bond strength increases, there is more efﬁcient overlap between molecular

orbitals that gives rise to a larger bandgap between bonding and antibonding MOs

(i.e., valence and conduction bands of the inﬁnite lattice, respectively). For a

particular Group 13 metal, as one moves down the Group 15 Period, the bonding

interaction between III–V elements will become weaker through the interaction of

more diffuse atomic orbitals. For instance, the bond strengths of Ga–N and Ga–As

bonds are 98.8 and 50.1 kcal mol

1

, respectively. The larger bandgap for GaN

relative to GaAs translates to a short wavelength (blue color) of emitted light that is

observed.

Most white LEDs employ a semiconductor chip emitting at a short wavelength

(blue), and a wavelength converter that absorbs light from the diode and undergoes

secondary emission at a longer wavelength. Such diodes emit light of two or more

wavelengths, that when combined, appear as white. The most common wavelength

converter materials are termed phosphors (e.g., ZnS – Figure 4.55), which exhibit

luminescence when they absorb energy from another radiation source. Typical LED

phosphors are present as a coating on the outside of the bulb, and are composed of an

inorganic host substance (e.g., yttrium aluminum garnet, YAG) containing an

optically active dopant (e.g., Ce). Use of such a single-crystal phosphor produces

a yellow light, upon combination with blue light gives the appearance of white.

A similar result has recently been produced through use of CdSe nanoparticles (see

Table 4.7. Bandgaps of III–V Semiconductors

Semiconductor Bandgap (eV)

AlN 6.02

AlP 2.45

GaN 3.50

GaP 2.35

GaAs 1.35

GaSb 0.67

InN 1.95

InP 1.27

InAs 0.36

4.3. Light-Emitting Diodes: There is Life Outside of Silicon! 325

Chapter 6).

[83]

White LEDs may also be made by coating near ultraviolet (NUV)

emitting LEDs with a mixture of europium-based red and blue emitting phosphors,

plus green emitting copper- and aluminium-doped zinc sulﬁde (ZnS:Cu,Al). It is

also possible for LEDs to emit white light without the use of phosphors. For

instance, homo epitaxially grown ZnSe crystals simultaneously emit blue light

from the ﬁlm, and yellow light from the ZnSe substrate.

Recently, there has been much interest in organic light-emitting diodes (OLEDs).

Flat-panel televisions, cellular phones, and digital cameras are already beginnin g to

employ this technology; it is only a matter of time before the “holy grail” of ﬂexible

display screens and luminous fabric s are produced. OLED displays offer many

beneﬁts relative to standard CRTs and LCDs such as enhanced brightness, lower

power consumption, and wider viewing angles.

The multilayered structure and electroluminescent mechanism of OLEDs is illu-

strated in Figure 4.78. Depending on whether small organic molecules or long

Figure 4.78. Multilayered structure of OLEDs/PLEDs. Also shown are the relative energy levels for

individual layers; light is emitted as a result of the radiative recombination of electron-hole pairs.

326 4 Semiconductors

repeating-unit polymers are used (Figure 4.79), the diodes are referred to as OLEDs

or PLEDs, respectively. Under positive current, electrons and holes are injected into

the emissive layer from opposite directions – from the cathode and anode, respec-

tively. The metal cathode is usually an alkaline earth or Al, which readily release a

Figure 4.79. Molecular structures of commonly used OLED/PLED materials. Shown are: (a) Alq

(tris

(quinoxalinato)Al (III)) used as an electron-transport material; (b) DIQA (diisoamylquinacridone) used as

an emissive dopant; (c) BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) used as an exciton/hole

blocking agent; (d) NPB (1,4-bis(1-napthylphenyl amino)biphenyl); (e) PFO (9,9-dioctylﬂuorene) used as

an emissive polymer in PLEDs; (f) PEDOT–PSS (poly-3,4-ethylenedioxythiophene–polystyrene

sulfonate) used as a hole transport material in PLEDs.

4.3. Light-Emitting Diodes: There is Life Outside of Silicon! 327