7.2 Patterning and Assembly
217
known to feature inherent photo- and thermal stability and excellent solution processability. In fact, light-emitting diodes based on cQDs had been demonstrated few
years ago [28].
Currently, efforts are driven to achieve high-brightness, high-contrast, highresolution television screens based on cQDs as emitters. Owing to the high colour
purity and relatively easy production techniques, they became attractive for application in screens. For mass production, dispersed particles could be directly inkjet
printed to form pixel-wise arranged active media of the device, to give a practical
example. Based on improvements in the efficiency and operation lifetime of quantumdot LEDs (QLEDs), ultra-thin and flexible, wide-colour-gamut, large-area, energysaving, and cost-effective displays are expected from their commercialisation (for
details, see [27]).
The only (obvious) drawback is that most useful core/shell cQDs are composed
of heavy metals. This renders the hunt for ecologically-friendly siblings, which also
perform well and are durable, a remaining challenge in order to enable the wider
employment of these nanoparticles in applications. Despite their ecological drawbacks, cQDs such as CdSe/ZnS or similar heterostructure clusters (and increasingly
perovskite cQDs [29, 30]) are still used in research to explore their advantages for
optoelectronic device concepts.
Remarkably, the else famously-known as indirect-gap material silicon (an
archetype lecture example for a bad emitter, although the role of defect states is completely ignored for simplicity) has become an attractive platform for optoelectronics
in the form of nanoparticles, which have successfully ended up in photodetection
and LED device schemes [31–36]. This is easily understood, as the strong spatial
localisation leads to a smearing out of the energy states in the phase space due to
Heisenberg’s uncertainty relation ΔxΔk ≥ 1. Thereby, the harsh optical selection
rules applying to bulk band structures are washed out due to the strong confinement
effects in colloidal nanocrystals.
Recently, hybrid systems of 2D materials decorated by nanocrystals gained popularity for improved photodetection and sensing applications (see for instance [23,
37–42]). For instance, monolayer-based field-effect transistors (FET) can be decorated with cQDs in order to functionalise the active region for sensitive detection of
visible to near-infrared light, as was done for a graphene FET in [23] for a study of
gate-tunable Förster energy transfer from cQDs to graphene (Fig. 7.3, also see Fig.
2.8 in Chap. 2). From a dispersion of less than 6-nm-sized CdSe/ZnS cQDs with
oleic acid (OA) surface ligands, which are commercially available, a small amount
of dots was drop-casted on the graphene channel of the device and the solvent was
evapourated. The ligand type determines whether the clusters can bind covalently
or not. To limit the modification of graphene’s electrical properties, the ligands’ end
group was chosen to be a methyl group. If necessary, for a given cQD dispersion, a
ligand exchange can be performed in the solution by which, for instance, the length
of the ligands and, thereby, the mean distance of cQDs to each other can be adjusted.
In another example, a hybrid structure of 2D layered GaTe with gold nanoparticles was studied and demonstrated to be useful for an ultrasensitive detection of
aromatic molecules [42]. Gold nanoparticles were grown directly onto mechanically-
217
known to feature inherent photo- and thermal stability and excellent solution processability. In fact, light-emitting diodes based on cQDs had been demonstrated few
years ago [28].
Currently, efforts are driven to achieve high-brightness, high-contrast, highresolution television screens based on cQDs as emitters. Owing to the high colour
purity and relatively easy production techniques, they became attractive for application in screens. For mass production, dispersed particles could be directly inkjet
printed to form pixel-wise arranged active media of the device, to give a practical
example. Based on improvements in the efficiency and operation lifetime of quantumdot LEDs (QLEDs), ultra-thin and flexible, wide-colour-gamut, large-area, energysaving, and cost-effective displays are expected from their commercialisation (for
details, see [27]).
The only (obvious) drawback is that most useful core/shell cQDs are composed
of heavy metals. This renders the hunt for ecologically-friendly siblings, which also
perform well and are durable, a remaining challenge in order to enable the wider
employment of these nanoparticles in applications. Despite their ecological drawbacks, cQDs such as CdSe/ZnS or similar heterostructure clusters (and increasingly
perovskite cQDs [29, 30]) are still used in research to explore their advantages for
optoelectronic device concepts.
Remarkably, the else famously-known as indirect-gap material silicon (an
archetype lecture example for a bad emitter, although the role of defect states is completely ignored for simplicity) has become an attractive platform for optoelectronics
in the form of nanoparticles, which have successfully ended up in photodetection
and LED device schemes [31–36]. This is easily understood, as the strong spatial
localisation leads to a smearing out of the energy states in the phase space due to
Heisenberg’s uncertainty relation ΔxΔk ≥ 1. Thereby, the harsh optical selection
rules applying to bulk band structures are washed out due to the strong confinement
effects in colloidal nanocrystals.
Recently, hybrid systems of 2D materials decorated by nanocrystals gained popularity for improved photodetection and sensing applications (see for instance [23,
37–42]). For instance, monolayer-based field-effect transistors (FET) can be decorated with cQDs in order to functionalise the active region for sensitive detection of
visible to near-infrared light, as was done for a graphene FET in [23] for a study of
gate-tunable Förster energy transfer from cQDs to graphene (Fig. 7.3, also see Fig.
2.8 in Chap. 2). From a dispersion of less than 6-nm-sized CdSe/ZnS cQDs with
oleic acid (OA) surface ligands, which are commercially available, a small amount
of dots was drop-casted on the graphene channel of the device and the solvent was
evapourated. The ligand type determines whether the clusters can bind covalently
or not. To limit the modification of graphene’s electrical properties, the ligands’ end
group was chosen to be a methyl group. If necessary, for a given cQD dispersion, a
ligand exchange can be performed in the solution by which, for instance, the length
of the ligands and, thereby, the mean distance of cQDs to each other can be adjusted.
In another example, a hybrid structure of 2D layered GaTe with gold nanoparticles was studied and demonstrated to be useful for an ultrasensitive detection of
aromatic molecules [42]. Gold nanoparticles were grown directly onto mechanically-