In this thesis, we introduce three new approaches for solving the single source shortest path (SSSP) problem in nearly acyclic directed graphs, and algorithms based on these approaches. In the first approach, we extend a technique of strongly connected components (sc-components) decomposition by Takaoka [23], and the generalized decomposition approach is called a higher-order decomposition. According to Takaoka's definition of acyclicity, the degree of cyclicity of a graph G, cyc(G), is defined by the maximum cardinality of the strongly connected components of G. Based on the higher-order decomposition, we give a generalization of Takaoka's definition of acyclicity. That is, the degree of cyclicity cych(G) is the maximum cardinality of the hth order strongly connected components of G, where h is the number of times that the graph has been decomposed. Then, the original definition introduced by Takaoka [23] can be presented as: The degree of cyclicity cyc(G) is the maximum cardinality of the 1th order strongly connected components of G. The second approach presents a new method for measuring acyclicity based on modifications to two existing methods. In the new method, we decompose the given graph into a 1-dominator set, which is a set of acyclic subgraphs, where each sub-graph is dominated by one trigger vertex. Meanwhile we compute sc-components of a degenerated graph derived from triggers. Using this preprocessing, we can efficiently compute the single source shortest paths (SSSPs) for nearly acyclic graphs in O(m + r logl ) time, where r is the size of the 1-dominator set, and l is the size of the largest sc-component. In the third approach, we modify the concept of a 1-dominator set to that of a 1-2-dominator set, and achieve O(m + r²) time to compute a 1- 2-dominator set in a graph. Each of acyclic sub-graphs obtained by the 1-2-dominator set are dominated by one or two trigger vertices cooperatively. Such sub-graphs are potentially larger than those decomposed by the 1-dominator set. Thus fewer trigger vertices are needed to cover the graph, that is, rʹ ≤ r, where rʹ is the number of triggers in the 1-2-dominator set. When rʹ is much smaller than r, we can efficiently compute SSSPs in O(m + rʹlogrʹ) time.

Title page, table of contents and abstract only. The complete thesis in print form is available from the University of Adelaide Library. / Internet Protocol multicast has been standardised since the late 1980's, but is yet to be extensively deployed by most Internet Service Providers. Many organisations are not willing to bear the additional router CPU load and memory requirements that multicast entails, and the IP multicast suite of protocols requires deployment on every router spanned by the multicast group to operate. Additionally, these protocols are predominantly designed for the general case of multiple-source, multiple-receiver transmission and can be complex and inefficient to use in simpler scenarios. Single-source streaming of multimedia on the Internet is rapidly becoming a very popular application, and is predominantly being served by content providers using simultaneous unicast streams. A multicast transmission protocol designed for this application that can operate without requiring a widely deployed IP multicast infrastructure has the potential to save content-providers and network service providers significant amounts of bandwidth. This protocol should provide packet duplication and forwarding capabilities on routers in the network, rather than pushing this functionality to the receivers themselves, requiring them to become part of the multicast infrastructure. We describe Lorikeet, a new protocol for the multicast distribution of multimedia streams from a single source. This protocol builds its multicast tree from the source, discovering routers that support the protocol in the network and using them to provide branching in the tree. The tree itself is managed in a decentralised fashion, with joining receivers finding parent routers through a limited, recursive search of the tree. On a participating node, information about the tree's structure is limited to the addresses of that node's children and its path through the tree back to the source. Unlike most other multicast protocols, a new receiver is connected to the tree using its forward path from the source and packets are delivered through the tree via hop-by-hop delivery over unicast connections between nodes. Lorikeet also actively maintains the tree structure using a localised rearrangement algorithm triggered by a topological change in the tree structure. This rearrangement allows the tree to remain efficient in the face of changes to the receiver population, which can change the shape of the tree over time. Lorikeet is designed to operate with no further protocol support than that provided by existing Internet unicast protocols. It requires none of the standard IP multicast infrastructure, such as Class D group addressing. Its use of unicast connections between nodes allows it to be deployed incrementa.lly on the network, and its behaviour will degrade to simultaneous unicast when no routers that support the protocol are present at all. However, significant performance gains can be achieved even when there are only a few supporting routers present in the network: Lorikeet produces trees with half the cost of a unicast tree when just 10% of routers are Lorikeet-capable. Lorikeet's tree construction and rearrangement algorithms generate multicast trees of comparable total cost to those created by algorithms of considerably higher message complexity, such as those that employ exhaustive searches of the tree during joins. We develop the Lorikeet protocol from a set of requirements based on its target application and the properties of the current Internet. After describing the protocol's behaviour, we analyse its message complexity and its performance in terms of tree cost. We also analyse several other multicast protocols from the research literature, comparing their performance to that of Lorikeet in both complete deployment and incremental deployment scenarios. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1283785 / Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Electronic Engineering, 2007

There is much interest in the electronic potential of ‘nano’-semiconductors. The avenue of research pursued in this project was in inorganic analogues of graphene, namely metal chalcogenides MxEy (M = metal, E = S, Se, Te, x ≠ y = integer value). Thin films of these materials have been used in solar cells, ambient thermoelectric generators and IR detectors, due to their interesting properties, such as: optoelectronics, magnetooptic, piezoelectric, thermoelectric and photovoltaic, as well as electrical conductivity. The key issues with the use of these materials are the formation of controlled films, especially in terms of stoichiometry, crystallinity and uniformity, and also the precursor system used. The aim of this research was to synthesise and isolate novel precursor compounds for use in the deposition of metal sulfide thin films (for use with molybdenum and tungsten). The potential viability of the compounds as single source precursors (ssp) was judged following ThermoGravimetric Analysis (TGA). The compounds were also subjected to analysis using NMR (1H, 13C and 31P where applicable), infrared and UV-Vis spectroscopy, as well as elemental analysis. Cadmium sulfide (CdS) is one of the key direct band gap II-VI semiconductors, having vital optoelectronic applications for laser light-emitting diodes, and optical devices based on non-linear properties. The ratio of these films should ideally be 1:1, however, during the formation of cadmium sulfide films, particularly at elevated temperatures, a common problem encountered is the production of sulfur deficient films. These films have a formula consistent with 〖Cd〗_x S_y, where x is an integer value greater than y, but the sulfur deficiency is generally no greater than 10 %. In order to correct this sulfur deficiency, it was decided to investigate deposition making use of both a ssp and an additional sulfur source, with the aim of producing uniform films with 1:1 Cd:S.Molybdenum disulfide films have been deposited previously from multi source precursors and more recently using ssp. In this project MoS2 was deposited using novel ssps in both LP and AACVD on a variety of substrates with the aim of producing uniform thin films and assessing any differences in the morphology of the deposition. This work was continued with the deposition of WS2 and MoxW1-xS2 from ssps which had not been reported previously. The films deposited were analysed using XRD, SEM, EDX (when available) and Raman spectroscopy.

Boron arsenide is a compound main group semiconductor with a theoretical band gap in the range of 1.1 to 1.6 eV. Despite this ideal band gap, experimental studies of boron arsenide are very limited. In the present work, single source precursors with covalent bonds between boron and arsenic and labile ligands have been designed and synthesized. These precursors underwent thermal or chemical treatment to produce boron arsenide materials. Boron arsenide has also been prepared as a thin layer deposited on a boron substrate and a p-type photoelectrode was prepared from this material. The structure of the product was identified on the basis of X-ray diffraction and scanning electron microscopy, and the surface composition was determined by means of X-ray photoelectron spectroscopy. The electrode was found to be photoactive under both visible and UV-visible light irradiation and displayed a photocurrent of approximately 0.1 mA/cm² under UV-visible light irradiation at an applied potential of -0.25 V vs. Ag/AgCl. The valence band was estimated to be -5.1 eV. The indirect band gap, as determined from incident photo-to-electron conversion efficiency plots, is 1.46 eV. An ester-fuctionalized salophen aluminum complex that features a polymerizable bithiophene as the ester R group has been designed and synthesized. Metallopolymers of this type offer the additional advantages of processability and uniformity of the resulting films. The new salophen complex exhibited emission in the blue region at 491 nm with a quantum yield of 8.19%, which is significantly larger than that of the isolated ligand. Electropolymerization of this complex on a platinum button electrode resulted in the formation of an electrically conductive polymer that is also ionically conductive at low scan rates. In the polymeric form, the emission wavelength was found to be red-shifted to 505 nm. / text

The work presented in this thesis reports the use of a series of novel thiobiuret metal complexes [M(SON(CNiPr2)2)n] (M = Cu, Ni, Fe, Zn, Cd or In; n = 2 or 3) for the first time as single source precursors for the colloidal synthesis of metal sulfide nanoparticles. Other single source precursor(s) were also used for the synthesis of CdSe, CdS, CdSe/CdS core/shell, CdSeS alloys and Cu2-xS nanoparticles in microfluidic reactors. Thermolysis experiments of [Cu(SON(CNiPr2)2)2] using only oleylamine produced Cu7S4 nanoparticles as a mixture of monoclinic and orthorhombic phases. Pure orthorhombic Cu7S4 nanoparticles were obtained when a solution of precursor in octadecene was injected into hot oleylamine whereas, Cu1.94S nanoparticles were obtained when a solution of the precursor in oleylamine was injected into hot dodecanethiol. The thermolysis of [Ni(SON(CNiPr2)2)2] gave Ni3S4 in all cases except when precursor solution in oleylamine was injected into hot octadecene which produced NiS nanoparticles. The thermolysis of [Fe(SON(CNiPr2)2)3] in oleylamine/oleylamine produced Fe7S8 nanoparticles but other combinations, in most cases, gave amorphous material. Thermolysis of [Zn(SON(CNiPr2)2)2] in oleylamine produced spherical ZnS nanoparticles. Particles with size smaller than 4.3 nm had a cubic phase, whereas the particles with size larger than 4.3 nm had a hexagonal crystal structure as suggested by the selected area electron diffraction. Powder X-Ray diffraction showed that the CdS nanoparticles obtained from the thermolysis of [Cd(SON(CNiPr2)2)2] in oleylamine were cubic under all reaction conditions except when dodecanethiol was used as an injection solvent which produced hexagonal CdS. β-In2S3 were synthesized from the thermolysis of [In(SON(CNiPr2)2)3]. Transmission electron microscopy showed that the copper, nickel and iron sulfide nanoparticles had various morphologies such as spherical, hexagonal disks, trigonal disks, rods or wires; depending on the reaction temperature, concentration of the precursor, the growth time and the solvent/capping agent combination. The zinc and cadmium sulfide nanoparticles were mostly spherical whereas the indium sulfide nanoparticles were produced in the form of ultra-thin (< 1.0 nm) nanorods or nanowires. ZnxCd1-xS and CuInS2 nanoparticles were synthesised from the 1,1,5,5-tetra-iso-propyl-4-thiobiureto complexes of Zn, Cd and Cu, In, respectively. Powder X-Ray diffraction showed that the obtained ZnxCd1-xS nanoparticles are cubic under all reaction conditions. The ZnxCd1-xS nanoparticles had an average diameter between 3.5 to 6.4 nm as shown by transmission electron microscopy. The optical properties of the ZnxCd1-xS nanoparticles were highly dependent on the ZnS to CdS precursor ratio and the solvents/capping agents. Chalcopyrite (tetragonal), wurtzite (hexagonal) or a mixture of both CuInS2 nanoparticles were obtained depending on the reaction conditions. TEM showed that the CuInS2 nanoparticles could be synthesised with different morphologies (spherical, hexagonal, trigonal or cone). Luminescent CuInS2 nanoparticles were obtained only in the absence of oleylamine. [Cd(S2CNMenHex)2], [Cd(Se2P(iPr)2)2] and [Cu(SON(CNiPr2)2)2] were used as single source precursor(s) for the synthesis of CdS, CdSe, CdSe/CdS core/shell, CdSeS alloys and Cu2-xS in microfludic reactor. The CdS nanoparticles were in size range of 5.0 to 8.0 nm whereas the CdSe nanoparticles were ultra small (ca. 2 nm) with blue luminescence. The CdSe/CdS core/shell and the CdSeS alloys were bluish green or green luminescent depending on their size. The copper sulfide nanoparticles were found to be monoclinic Cu7S4 or monoclinic Cu7S4 with minor impurities of rhombohedral Cu9S5 depending on the reaction conditions.

Metal chalcogenides (sulfides, selenides and tellurides) are materials of current interest due to their peculiar properties such as optoelectronic, magnetooptic, thermoelectric and piezoelectric displays. These semiconducting materials have potential applications in solar cell devices, infrared detectors and ambient thermoelectric generators. Previously, these materials have been deposited by multiple-source precursor route with several problems associated with this technique. This work describes the synthesis of metal complexes (Zn, Cd, Fe, Ni, Pd, Pt) using the imidodichalcogenodiphosphinate ligand (Woollins ligand). Their thermal decomposition together with structural and spectroscopy analysis was carried out. The complexes were used as single source precursors for the deposition of cadmium selenide, cadmium phosphide, cadmium sulfide, zinc selenide, iron selenide and the tellurides of nickel, palladium, platinum and iron as thin films and powders. These were deposited by AACVD and pyrolysis. The deposited thin films and powders were characterised by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energy dispersive analysis of X-rays (EDAX), X-ray photoelectron spectroscopy (XPS) and superconducting quantum interference device (SQUID). The cadmium complexes [Cd{iPr2P(Se)NP(Se)iPr2}2] and [Cd{iPr2P(S)NP(Se)iPr2}2] deposited the mixture of hexagonal CdSe and monoclinic Cd2P3 films at the flow rate of 160 sccm at 475 and 500 °C. At the flow rate of 240 sccm, only hexagonal CdSe was deposited from [Cd{iPr2P(Se)NP(Se)iPr2}2] at all temperatures. Hexagonal CdS and the mixture of orthorhombic Cd6P7/cubic Cd7P10 were deposited from [Cd{iPr2P(S)NP(S)iPr2}2]. The zinc complexes [Zn{iPr2P(Se)NP(Se)iPr2}2] and [Zn{iPr2P(S)NP(Se)iPr2}2] both deposited cubic ZnSe at all temperatures with the flow rates of 160 and 240 sccm. The iron complexes [Fe{(SePPh2)2N}2] and [Fe{(SePPh2NPPh2S)2N}2] deposited orthorhombic FeSe2 mixed with monoclinic Fe3Se4 by pyrolysis at 500 and 550 °C. An unresolved pattern was observed from the complex [Fe{(SePPh2NPPh2S)2N}2] at 550 °C. XPS analysis of the deposited FeSe2 showed the surface oxidation of the material, while the magnetic measurements on the sample using SQUID confirmed its ferromagnetic properties. The telluride complexes of nickel, palladium, platinum and iron deposited the metal telluride respectively as: hexagonal NiTe, hexagonal PdTe, hexagonal PtTe2 (mixed with rhombohedral PtTe) and hexagonal FeTe2. Conductivity studies on NiTe and PdTe revealed them to be insulators, while the magnetic measurements on FeTe2 indicated its antiferromagnetic behaviour.

Thesis (PhD (Chemistry and Polymer Science))--Stellenbosch University, 2008. / The successful preparation and structural characterization of a number of N,N-dialkyl-N’-benzoyl(thio)selenourea ligands is described; where the intermolecular interactions are characterized by the presence of Resonance Assisted Hydrogen Bonding (RAHB), π- π interactions between neighbouring benzene residues only being evident amongst the longer alkyl chain derivatives. The first structural characterization of an asymmetrically substituted N,N-dialkyl- N’-benzoylselenourea ligand reveals an increased stability of the Z isomer in the solid state, this being reflected by the sulfur analogue. Attempts to synthesise N,N-dicyclohexyl-N’-benzoylselenourea led to the isolation and structural characterization of a novel 1,3,5-oxaselenazine salt and dicyclohexylaminobenzoate. The first structural characterization of a “bipodal” N,N-dialkyl-N’-benzoylselenourea ligand, 3,3,3’,3’-tetrabutyl-1,1’- isophthaloylbis(selenourea), reveals RAHB in the crystal lattice similar to that exhibited by the “monopodal” analogue, N,N-dibutyl-N’-benzoylselenourea. The successful complexation of the N,N-dialkyl-N’-benzoyl(thio)selenourea ligands to a number of different transition metal ions is reported allowing the preparation of several potential single source precursors. Coordination through the O and Se/S donor atoms to Pd(II) results in the formation of square planar metal complexes, with a cis conformation, several of which could be structurally characterized. In particular, the first structural elucidation of an asymmetrically substituted N,N-dialkyl-N’-benzoylselenourea metal complex, cis-bis(N-benzyl-N-methyl-N’- benzoylselenoureato)palladium(II) indicates the increased stability of the EZ isomer in the solid state. Structural elucidation of the novel (N,N-diphenyl-N’-benzoylselenoureato)cadmium(II) reveals a bimetallic complex in the solid state, where the expected 2:1 ligand : metal ratio is maintained, and the two Cd(II) centres are 5 and 6 coordinated, with O and Se donor atoms. Multinuclear Nuclear Magnetic Resonance (NMR) Spectroscopy has been employed in the thorough characterisation of the potential single source precursors, 77Se NMR spectroscopy indicating a decreased shielding of the 77Se nucleus as the “hardness” of the central metal ion increases i.e. Pd(II) > Zn(II) > Cd(II). Use of 113Cd NMR spectroscopy indicates the preferential binding of N,N-diethyl-N’- benzoylselenourea to Cd(II) over that of its sulfur analogue, and initial studies suggest a form of chelate metathesis taking place in solution. 31P NMR spectroscopy is used to gain insight into the formation of cis-bis(N,N-diethyl-N’- benzoylselenoureato)Pt(II). Thermolysis of (N,N-diethyl-N’-benzoylselenoureato)cadmium(II) and its sulfur analogue led to the successful synthesis of CdSe and CdS quantum dots respectively, where thermolysis over a range of temperatures allows a degree of size control over the resulting nanoparticles. The effect of precursor alkyl chain length on nanoparticle morphology was investigated for both the N,N-dialkyl-N’-benzoylthio- and –selenoureas. A correlation between the two for the (N,N-dialkyl-N’-benzoylselenoureato)Cd(II) complexes is described and possible growth mechanisms are discussed. Preliminary investigations into the use of other N,N-dialkyl-N’-benzoyl(thio)selenourea metal complexes as single source precursors reveal that both (N,N-diethyl-N’-benzoylselenoureato)Zn(II) and its sulfur analogue show potential as single source precursors for the formation of ZnO and ZnS nanoparticles respectively. Initial studies into the use of N,N-dialkyl-N’-benzoyl(thio)selenourea metal complexes as single source precursors for the synthesis of core-shell nanoparticles is briefly described. The Aerosol Assisted Chemical Vapour Deposition (AACVD) of several N,N-dialkyl-N’-benzoyl(thio)selenourea metal complexes is reported, where both (N,N-diethyl-N’-benzoylselenoureato)Cd(II) and its sulfur analogue allow the deposition of crystalline CdSe and CdS respectively. The AACVD of (N,N-diethyl-N’- benzoylselenoureato)Zn(II) leads to the deposition of crystalline ZnSe, ZnS being deposited by (N,N-diethyl-N’-benzoylthioureato)Zn(II). The deposition of heazelwoodite (Ni3S2) with varying morphologies results from the AACVD of cis-bis(N,N-diethyl-N’-benzoylthioureato)Ni(II). Thermal annealing of the amorphous material deposited by the AACVD of cis-bis(N,N-diethyl-N’-benzoylthioureato)Pd(II), allows the formation of highly crystalline palladium. The deposition of metallic platinum using cis-bis(N,N-diethyl-N’-benzoylthioureato)Pt(II) is described as well as the deposition of crystalline Pd17Se15 from cis-bis(N,N-diethyl-N’-benzoylselenoureato)Pd(II). This, to the best of our knowledge, is the first time that AACVD has been performed, using the N,N-dialkyl-N’- benzoyl(thio)selenourea metal complexes as single source precursors, in addition, we believe it to be the first time that palladium selenide has been deposited using the AACVD technique.

In this thesis, the feasibility of employing organometallic single-source precursors in the preparation of advanced transition metal pnictide materials such as colloidal nanoparticles and films has been investigated. In particular, the ternary FeMnP phase was targeted as a model for preparing advanced heterobimetallic phosphide materials, and the iron-rich Fe 3 P phase was targeted due to its favorable ferromagnetic properties as well as the fact that the preparation of advanced Fe 3 P materials has been elusive by commonly used methods. Progress towards the synthesis of advanced Fe 2-x Mn x P nanomaterials and films was facilitated by the synthesis of the novel heterobimetallic complexes FeMn(CO) 8 (μ-PR 1 R 2 ) (R 1 = H, R 2 = H or R 1 = H, R 2 = Ph), which contain the relatively rare μ-PH2 and μ-PPhH functionalities. Iron rich Fe 2-x Mn x P nanoparticles were obtained by thermal decomposition of FeMn(CO) 8 (μ-PH 2 ) using solution-based synthetic methods, and empirical evidence suggested that oleic acid was responsible for manganese depletion. Films containing Fe, Mn, and P with the desired stoichiometric ratio of 1:1:1 were prepared using FeMn(CO) 8 (μ-PH 2 ) in a simple low-pressure metal-organic chemical vapor deposition (MOCVD) apparatus. Although the elemental composition of the precursor was conserved in the deposited film material, spectroscopic evidence indicated that the films were not composed of pure-phase FeMnP, but were actually mixtures of crystalline FeMnP and amorphous FeP and Mn x O y . A new method for the preparation of phase-pure ferromagnetic Fe 3 P films on quartz substrates has also been developed. This approach involved the thermal decomposition of the single-source precursors H 2 Fe 3 (CO) 9 PR (R = t Bu or Ph) at 400 °C. The films were deposited using a simple home-built MOCVD apparatus and were characterized using a variety of analytical methods. The films exhibited excellent phase purity, as evidenced by X-ray diffraction, X-ray photoelectron spectroscopy, and field-dependent magnetization measurements, the results of which were all in good agreement with measurements obtained from bulk Fe 3 P. As-deposited Fe 3 P films were found to be amorphous, and little or no magnetic hysteresis was observed in plots of magnetization versus applied field. Annealing the Fe 3 P films at 550 °C resulted in improved crystallinity as well as the observation of magnetic hysteresis.

xx, 153 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / In the research area of Group 13 hydroxide clusters, progress is often hampered by difficult and inefficient synthetic procedures. This has greatly limited the numerous potential applications of Group 13 hydroxide compounds, many of which require large amounts of material. Most relevant to this dissertation is their application as precursors for high quality amorphous metal oxide thin films. Addressing this issue, this dissertation presents a series of Group 13 containing hydroxide compounds of general formula [M 13 (μ 3 -OH) 6 (μ-OH) 18 (H 2 O) 24 ](NO 3 ) 15 which are generated through an efficient, scalable synthetic procedure. Throughout this dissertation, the compounds are generally referred to by their metal content, i.e. [Ga 13 (μ 3 -OH) 6 (μ-OH) 18 (H 2 O) 24 ](NO 3 ) 15 is designated as Ga 13 . Chapter I reviews the literature of inorganic and ligand-supported Group 13 hydroxide compounds with the aim of identifying common structural trends in metal composition and coordinating ligands. This summary is limited to clusters of aluminum, gallium, and indium. Chapter II describes in detail the synthesis and characterization of one such cluster, Al 13 . Following this in Chapter III is the description of the first heterometallic Group 13 hydroxide compound, Ga 7 In 6 , which along with Ga 13 was used as a precursor material for metal oxide thin films in collaboration with Professor Doug Keszler at Oregon State University. Chapter IV describes a series of six Ga/In compounds, as well as two Al/In compounds. Included in this chapter is an analysis of the heat-induced decomposition properties of the Ga/In clusters. Understanding such thermal decomposition is particularly relevant for the use of these compounds as precursor materials, as an annealing step is used to condense the films. Chapter V addresses the potential for post-synthetic modification of the compounds through metal and ligand exchange reactions, an area that also addresses the issue of solution stability of the structures Chapter VI describes the synthesis and characterization of related Group 13 compounds, including two infinite chain structures and additional heterometallic compounds. Lastly, Chapter VII concludes this dissertation and discusses potential areas of future research. This dissertation includes co-authored material and previously published results. / Committee in charge: Victoria DeRose, Chairperson, Chemistry; Darren Johnson, Member, Chemistry; James Hutchison, Member, Chemistry; Michael Haley, Member, Chemistry; Raghuveer Parthasarathy, Outside Member, Physics

Source localization with a wireless sensor network remains an important area of research as the number of applications with this problem increases. This work considers the problem of source localization by a network of passive wireless sensors. The primary means by which localization is achieved is through direction-finding at each sensor, and in some cases, range estimation as well. Both single and multiple-target scenarios are considered in this research. In single-source environments, a solution that outperforms the classic least squared error estimation technique by combining direction and range estimates to perform localization is presented. In multiple-source environments, two solutions to the complex data association problem are addressed. The first proposed technique offers a less complex solution to the data association problem than a brute-force approach at the expense of some degradation in performance. For the second technique, the process of signal classification is considered as another approach to the data association problem. Environments in which each signal possesses unique features can be exploited to separate signals at each sensor by their characteristics, which mitigates the complexity of the data association problem and in many cases improves the accuracy of the localization. Two approaches to signal-selective localization are considered in this work. The first is based on the well-known cyclic MUSIC algorithm, and the second combines beamforming and modulation classification. Finally, the implementation of a direction-finding system is discussed. This system includes a uniform circular array as a radio frequency front end and the universal software radio peripheral as a data processor. / Master of Science